arXiv subjects

Jingyang Zhao

Publications and source records attributed to Jingyang Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Constant-Factor Approximations for the Biased Traveling Salesman Problem and Its Path Variant

The metric traveling salesman problem (TSP) is a fundamental problem in combinatorial optimization that asks for a minimum-cost Hamiltonian cycle in a complete undirected graph with $N$ vertices, where edge costs satisfy the triangle inequality. The biased TSP (BTSP) is a natural generalization of the metric TSP, in which the vertex set is partitioned into $k$ subsets: edge costs satisfy the triangle inequality within each subset, while edges crossing the subsets may violate the triangle inequality by a certain bias factor. This model captures scenarios in which distances are metric only locally but may be distorted across different regions or types of vertices. In this paper, we study approximation algorithms for the BTSP and its path variant, the biased traveling salesman path problem (BTSPP), which asks for a minimum-cost Hamiltonian path. Previously, constant-factor approximation algorithms were known only for the case $k=2$. Whether the BTSP and the BTSPP admit constant-factor approximation algorithms for any fixed integer $k>2$ has remained an open question. We resolve this question affirmatively. We show that both the BTSP and the BTSPP admit $O(k)$-approximation algorithms with running time $N^{O(2^k k!)}$. Moreover, for the BTSPP with $k=2$, we propose an improved $(α+2+\varepsilon)$-approximation algorithm, where $α<3/2-10^{-34}$ is the best-known approximation ratio for the metric TSP, improving upon the previous ratio of $4$

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arXiv2026

Constant-Factor Approximation Algorithms for Weighted Budgeted Multisensor Sweep Coverage

Sweep coverage requires each point of interest (PoI) to be revisited by some mobile sensor at least once during every sweep period, and it underlies persistent patrolling, environmental monitoring, and infrastructure inspection. Given a set of $N$ PoIs, each associated with a nonnegative weight representing its importance, the weighted budgeted sweep coverage problem (WB-SCP) seeks routing trajectories for a budget of $M\leq N$ identical mobile sensors with a common maximum speed to maximize the total weight of the sweep-covered PoIs. For the unweighted special case in which every PoI has unit weight, the first constant-factor approximation algorithm achieves a ratio of $0.0116-O(\varepsilon)$ for any constant $\varepsilon>0$, and whether the WB-SCP itself admits such an algorithm has remained open. In this paper, we resolve this open question by proposing two constant-factor approximation algorithms for the WB-SCP. The first converts any bifactor $(α,β)$-approximation algorithm into a $β/(2α)$-approximation algorithm, yielding a ratio of $1/16=0.0625$. The second builds on a greedy maximum-coverage framework and improves the ratio to $1-e^{-1/2}-\varepsilon$ for any fixed $\varepsilon>0$. Both algorithms also improve the best-known ratio for the unweighted case, and the second extends to heterogeneous sensors with different speeds while preserving the same guarantee.

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arXiv2026

Polynomial $(2α+\varepsilon)$- and Quadratic $4$-Approximation Algorithms for the Biased Traveling Salesman Problem in Bipartitioned Networks

The traveling salesman problem (TSP) is one of the fundamental problems in combinatorial optimization that asks for a minimum-cost Hamiltonian cycle in a complete undirected graph with $N$ vertices. For the metric TSP, where edge costs satisfy the triangle inequality, a long line of work has produced constant-factor approximation algorithms, culminating in the best-known approximation ratio $α<3/2-10^{-36}$. The biased TSP (BTSP) generalizes the metric TSP to bipartitioned networks: the vertex set is partitioned into two subsets, edge costs satisfy the triangle inequality within each subset, while edges crossing the partition may violate it by a certain bias factor. This model captures heterogeneous routing scenarios in which distances are metric only locally but may be distorted across different regions or vertex types. Previously, the only known result for the BTSP was a $7/2$-approximation algorithm with running time $O(N^3)$. In this paper, we present two new approximation algorithms that significantly improve the state of the art. The first achieves an approximation ratio of $2α+\varepsilon$ (strictly less than $3$) in polynomial time $N^{O(1)}$ for any constant $\varepsilon>0$, where the exponent grows at least linearly with $1/\varepsilon$. The second achieves a $4$-approximation in only $O(N^2)$ running time, reducing the previous cubic-time bound by a full polynomial factor. These advances directly support emerging networked systems---including multi-tier 6G architectures and hybrid quantum-classical networks---where cross-region transitions incur biased and non-metric overheads.

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arXiv2026

Approximation and Inapproximability of Non-Preemptive Dial-a-Ride

The non-preemptive dial-a-ride problem (DaRP) asks for a minimum-cost route of a capacitated vehicle that serves a collection of pickup-and-delivery requests, without allowing objects to be transferred or temporarily unloaded. We study this problem and the closely related minimum-ratio Steiner $k$-Forest (S$k$F) in general symmetric metrics on $n$ locations. We develop a randomized polynomial-time algorithm with expected approximation ratio $\mathcal{O}(n^{1/4}\log^{11}n)$ for the minimum-ratio S$k$F. Our algorithm combines a deterministic approximation on hierarchically well-separated trees with randomized metric embeddings. Using this algorithm, we obtain expected $\mathcal{O}(n^{1/4}\log^{12}n)$-approximations for both the non-preemptive DaRP and the S$k$F. These guarantees improve on the previous $n^{0.3+\varepsilon}$-approximation ratios for the S$k$F and the non-preemptive DaRP, where $\varepsilon>0$ is any fixed constant. Complementing these algorithmic results, we establish the first superconstant inapproximability bounds for the non-preemptive DaRP. Under the Exponential Time Hypothesis (ETH), no deterministic polynomial-time algorithm achieves an $x^{1/(\log\log x)^c}$-approximation for some constant $c>0$, where $x$ denotes either the number of locations $n$ or the vehicle capacity $k$. Under the quantitative Gap-ETH, we rule out every deterministic polynomial-time $x^{o(1)}$-approximation. These bounds hold even on unit-edge stars with $n=(2+o(1))k$.

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arXiv2026

Approximating Unsplittable Capacitated Vehicle Routing: Analysis via Factor-Revealing LPs

The capacitated vehicle routing problem (CVRP) is one of the most extensively studied problems in combinatorial optimization. Given a depot and a set of customers with demands in a metric space, the objective is to find a set of tours, each starting and ending at the depot, operated by a capacitated vehicle located at the depot, such that all customers are served and the total travel cost is minimized. We consider the unsplittable variant, where the demand of each customer must be served entirely by a single tour. The previous best approximation ratio was about $α+1.694$, where $α$ is the approximation ratio for the traveling salesman problem. We propose improved approximation algorithms. First, we introduce a unified algorithmic framework that generalizes and integrates known linear programming (LP) rounding and matching-based approaches. We show that these techniques are complementary and can be combined to yield improved approximation ratios. Second, we develop a novel analytical framework based on customer demand discretization, which yields new structural bounds and, through factor-revealing LPs, achieves expected approximation ratios of about $α+1.647$ for the general-capacity case and $α+1.548$ for the fixed-capacity case. Finally, we derandomize our algorithms using a priori 2-tour sets and the method of conditional expectations while maintaining nearly the same approximation ratios.

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arXiv2026

Tight Approximations for Fixed-Order Vehicle Routing with Stochastic Demands

We study the vehicle routing problem (VRP) with stochastic demands, where each client has an independent demand with a known distribution, and its realized demand is observed only on the first visit. We consider a priori and restocking policies that fix the client order before observing demands and serve each client's entire demand before first visiting the next. Gupta, Nagarajan, and Ravi (Oper. Res., 2012) developed randomized a priori $(α+1)$-approximation and $(α+2)$-approximation algorithms for splittable and unsplittable deliveries, respectively, where $α$ is the approximation ratio of a metric traveling salesman problem (TSP) algorithm. We improve these approximation ratios to $2$ and $3$ by using their replenishment rules with a TSP tour constructed via matroid parity. We also obtain deterministic a priori $(2+\varepsilon)$-approximation and $(3+\varepsilon)$-approximation algorithms by compressing the distributions of cumulative demands to select an initial stock in polynomial time. All guarantees are relative to the general adaptive optimum. We then establish matching lower bounds of $2$ and $3$ that hold even for the more general class of randomized restocking policies. Finally, we apply our matroid-parity construction to improve approximation ratios for several VRP variants with deterministic demands.

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arXiv2026

A $(1+1/\sqrt{2})$-Approximation for the Multiple-Depot Traveling Salesman Problem

The metric traveling salesman problem (TSP) is a fundamental problem in combinatorial optimization that asks for a minimum-cost tour covering all clients in a metric graph. The metric multiple-depot TSP (MD-TSP) is a natural extension, where the graph contains depots and clients, and the objective is to compute a minimum-cost set of tours covering all clients, with each tour starting and ending at the same depot. When the number of depots is part of the input, an adaptation of the Christofides--Serdyukov heuristic yields an approximation ratio of $2$. In this paper, we introduce a $(1+1/\sqrt{2})$-approximation algorithm. Like the Christofides--Serdyukov heuristic, our algorithm first computes a rooted spanning forest (RSF), then a matching to correct its odd degrees, and finally obtains a solution by shortcutting. However, instead of using a minimum-cost RSF, we construct an RSF by a primal-dual algorithm for a natural cut relaxation. The algorithm grows rootless components and the component containing all depots at different rates, adding an edge when its dual constraint becomes tight. Vertex labels record the times at which clients first become connected to a depot. The two-speed growth provides a joint bound on the forest cost and two label-dependent terms that also arise in bounding the parity-correction cost. Balancing the coefficients of these two terms by setting both to $\sqrt{2}-1$ yields the claimed approximation ratio.

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arXiv2026

Improved Approximation Algorithms for the Multiple-Depot Split Delivery Vehicle Routing Problem

The multiple-depot split delivery vehicle routing problem is a challenging optimization problem with broad applications in logistics and transportation. The goal is to serve customers' demand using a limited fleet of capacitated vehicles stationed at multiple depots, allowing each customer's demand to be split and served by multiple vehicles, while minimizing the total travel cost. Parameterized by the number of depots, the previous best-known result was a slice-wise polynomial-time $6$-approximation algorithm (INFORMS J. Comput. 2023), and whether this ratio could be improved remained an open question. We resolve this question by proposing a fixed-parameter tractable (FPT) $(2α+1+\varepsilon)$-approximation algorithm for any constant $\varepsilon>0$, where $α<3/2$ denotes the best-known ratio for the traveling salesman problem (TSP). Our algorithm enumerates partitions of connected components formed by low-cost edges to construct a low-cost cycle cover, then extracts paths, and assigns them to vehicles through a minimum-cost flow method. The cycle-cover technique also yields an FPT $(α+\varepsilon)$-approximation for the multiple-depot TSP. We further propose a simple parameterized $5$-approximation algorithm based on the structural properties of vehicle capacities, which achieves polynomial running time for a specific setting that appears in existing benchmark instances. In addition, we develop a bi-factor approximation algorithm that balances minor vehicle capacity violations against reductions in travel cost or gains in computational efficiency. Finally, our computational experiments demonstrate that the proposed methods exhibit complementary strengths across various instance types and achieve competitive solution quality.

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arXiv2026

Xiaomi Auto World Model: A Joint World Model Integrating Reconstruction and Generation for Autonomous Driving

This report presents a unified technical system addressing the two core capabilities of world models for autonomous driving: world representation and world generation. For world representation, we propose WorldRec, a feed-forward reconstruction architecture driven by sparse scene queries. WorldRec initializes structured queries in 3D space, leveraging them to aggregate cross-view, cross-temporal features, thereby naturally enforcing spatial consistency across frames and yielding compact yet high-fidelity 3D Gaussian scene representations. For world generation, we propose WorldGen, a two-stage training framework of bidirectional pretraining followed by causal fine-tuning through three progressive stages (Teacher Forcing, ODE distillation, and DMD), enabling high-quality online causal video generation in as few as 4 denoising steps. Building on both modules, we further introduce the JWM, which deeply integrates WorldRec and WorldGen to achieve synergistic gains in generation stability, cross-frame consistency, and visual fidelity, providing a solid foundation for closed-loop simulation, data synthesis, and end-to-end training in autonomous driving.

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arXiv2026

Improved Approximations for Dial-a-Ride Problems

The multi-vehicle dial-a-ride problem (mDaRP) is a fundamental vehicle routing problem with pickups and deliveries, widely applicable in ride-sharing, economics, and transportation. Given a set of $n$ locations, $h$ vehicles of identical capacity $λ$ located at various depots, and $m$ ride requests each defined by a source and a destination, the goal is to plan non-preemptive routes that serve all requests while minimizing the total travel distance, ensuring that no vehicle carries more than $λ$ passengers at any time. The best-known approximation ratio for the mDaRP remains $\mathcal{O}(\sqrtλ\log m)$. We propose two simple algorithms: the first achieves the same approximation ratio of $\mathcal{O}(\sqrtλ\log m)$ with improved running time, and the second attains an approximation ratio of $\mathcal{O}(\sqrt{\frac{m}λ})$. A combination of them yields an approximation ratio of $\mathcal{O}(\sqrt[4]{n}\log^{\frac{1}{2}}n)$ under $m=Θ(n)$. Moreover, for the case $m\gg n$, by extending our algorithms, we derive an $\mathcal{O}(\sqrt{n\log n})$-approximation algorithm, which also improves the current best-known approximation ratio of $\mathcal{O}(\sqrt{n}\log^2n)$ for the classic (single-vehicle) DaRP, obtained by Gupta et al. (ACM Trans. Algorithms, 2010).

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arXiv2025

An Improved Approximation Algorithm for Maximum Weight 3-Path Packing

Given a complete graph with $n$ vertices and non-negative edge weights, where $n$ is divisible by 3, the maximum weight 3-path packing problem is to find a set of $n/3$ vertex-disjoint 3-paths such that the total weight of the 3-paths in the packing is maximized. This problem is closely related to the classic maximum weight matching problem. In this paper, we propose a $10/17$-approximation algorithm, improving the best-known $7/12$-approximation algorithm (ESA 2015). Our result is obtained by making a trade-off among three algorithms. The first is based on the maximum weight matching of size $n/2$, the second is based on the maximum weight matching of size $n/3$, and the last is based on an approximation algorithm for star packing. Our first algorithm is the same as the previous $7/12$-approximation algorithm, but we propose a new analysis method -- a charging method -- for this problem, which is not only essential to analyze our second algorithm but also may be extended to analyze algorithms for some related problems.

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arXiv2025

Parameterized Approximation Algorithms for TSP on Non-Metric Graphs

The Traveling Salesman Problem (TSP) is a classic and extensively studied problem with numerous real-world applications in artificial intelligence and operations research. It is well-known that TSP admits a constant approximation ratio on metric graphs but becomes NP-hard to approximate within any computable function $f(n)$ on general graphs. This disparity highlights a significant gap between the results on metric graphs and general graphs. Recent research has introduced some parameters to measure the ``distance'' of general graphs from being metric and explored Fixed-Parameter Tractable (FPT) approximation algorithms parameterized by these parameters. Two commonly studied parameters are $p$, the number of vertices in triangles violating the triangle inequality, and $q$, the minimum number of vertices whose removal results in a metric graph. In this paper, we present improved FPT approximation algorithms with respect to these two parameters. For $p$, we propose an FPT algorithm with a 1.5-approximation ratio, improving upon the previous ratio of 2.5. For $q$, we significantly enhance the approximation ratio from 11 to 3, advancing the state of the art in both cases. In addition, when $p$ (or $q$) is a constant, we obtain a better approximation ratio.

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arXiv2025

An Improved Approximation Algorithm for the Capacitated Arc Routing Problem

The Capacitated Arc Routing Problem (CARP), introduced by Golden and Wong in 1981, is an important arc routing problem in Operations Research, which generalizes the famous Capacitated Vehicle Routing Problem (CVRP). When every customer has a unit demand, the best known approximation ratio for CARP, given by Jansen in 1993, remains $\frac{5}{2}-\frac{1.5}{k}$, where $k$ denotes the vehicle capacity. Based on recent progress in approximating CVRP, we improve this result by proposing a $(\frac{5}{2}-Θ(\frac{1}{\sqrt{k}}))$-approximation algorithm, which to the best of our knowledge constitutes the first improvement over Jansen's bound.

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arXiv2025

Approximation Algorithms for the Cumulative Vehicle Routing Problem with Stochastic Demands

In the Cumulative Vehicle Routing Problem (Cu-VRP), we need to find a feasible itinerary for a capacitated vehicle located at the depot to satisfy customers' demand, as in the well-known Vehicle Routing Problem (VRP), but the goal is to minimize the cumulative cost of the vehicle, which is based on the vehicle's load throughout the itinerary. If the demand of each customer is unknown until the vehicle visits it, the problem is called Cu-VRP with Stochastic Demands (Cu-VRPSD). Assume that the approximation ratio of metric TSP is $1.5$. In this paper, we propose a randomized $3.456$-approximation algorithm for Cu-VRPSD, improving the best-known approximation ratio of $6$ (Discret. Appl. Math. 2020). Since VRP with Stochastic Demands (VRPSD) is a special case of Cu-VRPSD, as a corollary, we also obtain a randomized $3.25$-approximation algorithm for VRPSD, improving the best-known approximation ratio of $3.5$ (Oper. Res. 2012). For Cu-VRP, we give a randomized $3.194$-approximation algorithm, improving the best-known approximation ratio of $4$ (Oper. Res. Lett. 2013). Moreover, if each customer is allowed to be satisfied by using multiple tours, we obtain further improvements for Cu-VRPSD and Cu-VRP.

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arXiv2025

Improved Approximation Algorithms for Capacitated Vehicle Routing with Fixed Capacity

The Capacitated Vehicle Routing Problem (CVRP) is one of the most extensively studied problems in combinatorial optimization. Based on customer demand, we distinguish three variants of CVRP: unit-demand, splittable, and unsplittable. In this paper, we consider $k$-CVRP in general metrics and on general graphs, where $k$ is the vehicle capacity. All three versions are APX-hard for any fixed $k\geq3$. Assume that the approximation ratio of metric TSP is $\frac{3}{2}$. We present a $(\frac{5}{2}-Θ(\frac{1}{\sqrt{k}}))$-approximation algorithm for the splittable and unit-demand cases, and a $(\frac{5}{2}+\ln2-Θ(\frac{1}{\sqrt{k}}))$-approximation algorithm for the unsplittable case. Our approximation ratio is better than the previous results when $k$ is less than a sufficiently large value, approximately $1.7\times10^7$. For small values of $k$, we design independent and elegant algorithms with further improvements. For the splittable and unit-demand cases, we improve the approximation ratio from $1.792$ to $1.500$ for $k=3$, and from $1.750$ to $1.500$ for $k=4$. For the unsplittable case, we improve the approximation ratio from $1.792$ to $1.500$ for $k=3$, from $2.051$ to $1.750$ for $k=4$, and from $2.249$ to $2.157$ for $k=5$. The approximation ratio for $k=3$ surprisingly achieves the same value as in the splittable case. Our techniques, such as EX-ITP -- an extension of the classic ITP method, have the potential to improve algorithms for other routing problems as well.

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arXiv2025

The APX-hardness of the Traveling Tournament Problem

The Traveling Tournament Problem (TTP-$k$) is a well-known benchmark problem in sports scheduling, which asks us to design a double round-robin schedule such that each pair of teams plays one game in each other's home venue, no pair of teams plays each other on two consecutive days, each team plays at most $k$ consecutive home games or away games, and the total traveling distance of all the $n$ teams is minimized. TTP-$k$ allows a polynomial-time approximation scheme when $k=2$ and becomes APX-hard when $k\geq n-1$. In this paper, we reduce the gap by showing that TTP-$k$ is APX-hard for any fixed $k\geq3$.

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arXiv2025

An Improved Algorithm for a Bipartite Traveling Tournament in Interleague Sports Scheduling

The bipartite traveling tournament problem (BTTP) addresses inter-league sports scheduling, which aims to design a feasible bipartite tournament between two $n$-team leagues under some constraints such that the total traveling distance of all participating teams is minimized. Since its introduction, several methods have been developed to design feasible schedules for NBA, NPB and so on. In terms of solution quality with a theoretical guarantee, previously only a $(2+\varepsilon)$-approximation is known for the case that $n\equiv 0 \pmod 3$. Whether there are similar results for the cases that $n\equiv 1 \pmod 3$ and $n\equiv 2 \pmod 3$ was asked in the literature. In this paper, we answer this question positively by proposing a $(3/2+\varepsilon)$-approximation algorithm for any $n$ and any constant $\varepsilon>0$, which also improves the previous approximation ratio for the case that $n\equiv 0 \pmod 3$.

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arXiv2025

Enhanced Approximation Algorithms for the Capacitated Location Routing Problem

The Capacitated Location Routing Problem is an important planning and routing problem in logistics, which generalizes the capacitated vehicle routing problem and the uncapacitated facility location problem. In this problem, we are given a set of depots and a set of customers where each depot has an opening cost and each customer has a demand. The goal is to open some depots and route capacitated vehicles from the opened depots to satisfy all customers' demand, while minimizing the total cost. In this paper, we propose a $4.169$-approximation algorithm for this problem, improving the best-known $4.38$-approximation ratio. Moreover, if the demand of each customer is allowed to be delivered by multiple tours, we propose a more refined $4.091$-approximation algorithm. Experimental study on benchmark instances shows that the quality of our computed solutions is better than that of the previous algorithm and is also much closer to optimality than the provable approximation factor.

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