Gas Station

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

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

Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.

Note:
The solution is guaranteed to be unique.

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int[] diff = new int[gas.length];
        for(int i = 0; i < gas.length; i++){
            diff[i] = gas[i]-cost[i]; // add all gas available, and get what left in the next station. 
        }
       
        int leftGas = 0;
        int start = 0;
        int sum = 0;
        for(int i = 0; i < gas.length; i++){
            leftGas += diff[i];   // use leftGas at last to fill the "gap"
            sum += diff[i];
            if(sum < 0){
                start = i+1;
                sum = 0;
            }
        }
        if(leftGas < 0) return -1;
        else return start;
    }
}