Gas Station

Gas Station is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Greedy Patterns pattern, in the Gas Station Circuit subpattern.

Problem statement

There are n gas stations along a circular route, where the amount of gas at the ith station is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from the ith station to its next (i + 1)th station. You begin the journey with an empty tank at one of the gas stations.

Given two integer arrays gas and cost, return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1. If there exists a solution, it is guaranteed to be unique.

Example 1:

Input: gas = [1,2,3,4,5], cost = [3,4,5,1,2] Output: 3 Explanation: Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4 Travel to station 4. Your tank = 4 - 1 + 5 = 8 Travel to station 0. Your tank = 8 - 2 + 1 = 7 Travel to station 1. Your tank = 7 - 3 + 2 = 6 Travel to station 2. Your tank = 6 - 4 + 3 = 5 Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3. Therefore, return 3 as the starting index.

Example 2:

Input: gas = [2,3,4], cost = [3,4,3] Output: -1 Explanation: You can't start at station 0 or 1, as there is not enough gas to travel to the next station. Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4 Travel to station 0. Your tank = 4 - 3 + 2 = 3 Travel to station 1. Your tank = 3 - 3 + 3 = 3 You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3. Therefore, you can't travel around the circuit once no matter where you start.

Constraints:

- n == gas.length == cost.length

- 1 <= n <= 105

- 0 <= gas[i], cost[i] <= 104

- The input is generated such that the answer is unique.

Topics and companies

Topics: Array, Greedy.

Reported in interviews at Amazon, Microsoft, Apple, Hotstar, Google, Bloomberg, Citrix, C3 IoT.

How to practise Gas Station on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n), where n is the number of gas stations. We iterate through the gas and cost arrays once. time and O(1), as we use only a constant amount of extra space regardless of input size. space.

Open Gas Station in the code editor, or read the Gas Station editorial for a worked solution with its approach and complexity analysis.

Where Gas Station sits in the DSA pattern sheet

Problems related to Gas Station

Other problems that use the same Gas Station Circuit and Greedy Patterns techniques:

Preparing this workspace.