Leetcode-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.

Analysis:

The amount of gas left in tank is non-decreasing. If we cannot pass a station P (>x) strating from x, it is because we do not accmulate enough gas in the tank, so we start from x+1, x+2,...P-1, we will still not be able to pass P. Instead, we try to start from P+1, so that we have the maximum accumulative gas in the tank when reaching P.

If we cannot pass P (<x) starting from x, we already try P+1 (<=x), then we have no way to pass P, fail directly.

Solution:

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        if (gas.length==0) return -1;
        if (gas.length==1)
            if (gas[0]-cost[0]>=0) return 0;
            else return -1;

        int len = gas.length;
        int left = 0;
        int index = 0;
        while (index<len){
            left = 0;
            //Check whether the current station is the solution.
            boolean valid = true;
            int p = index;
            while (p!=(index+gas.length-1)%gas.length){
                if (left+gas[p]-cost[p]<0){
                    valid = false;
                    break;
                } else{
                    left += gas[p]-cost[p];
                    p = (p+1)%gas.length;
                }
            }
            if (valid && left+gas[p]-cost[p]>=0) return index;
            else if ((p+1)%gas.length<=index) return -1;
            else  index = p+1;
        }

        return -1;
    }
}