## 134. Gas Station

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 <= 104`
• `0 <= gas[i], cost[i] <= 104`

## Rust Solution

``````struct Solution;

impl Solution {
fn can_complete_circuit(gas: Vec<i32>, cost: Vec<i32>) -> i32 {
let n = gas.len();

let mut total = 0;
let mut cur = 0;
let mut idx = 0;

for i in 0..n {
total += gas[i] - cost[i];
cur += gas[i] - cost[i];

if cur < 0 {
idx = i + 1;
cur = 0;
}
}

if total >= 0 {
idx as i32
} else {
-1
}
}
}

#[test]
fn test() {
let gas = vec![1, 2, 3, 4, 5];
let cost = vec![3, 4, 5, 1, 2];
let res = 3;
assert_eq!(Solution::can_complete_circuit(gas, cost), res);
}
``````

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