# 0134. Gas Station

<https://leetcode.com/problems/gas-station>

## Description

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:**

* `gas.length == n`
* `cost.length == n`
* `1 <= n <= 105`
* `0 <= gas[i], cost[i] <= 104`

## ac

```java
class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int n = gas.length;
        int s = 0;
        while (s < n) {
            int k = 0;
            int gsum = 0;
            int csum = 0;
            while (gsum >= csum) {
                if (k > n) return s;
                gsum += gas[(s+k)%n];
                csum += cost[(s+k)%n];
                k++;
            }
            s = s + k;
        }
        return -1;
    }
}
```
