Gas Station

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

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from stationi 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.

思路：

• If car starts at A and can not reach B. Any station between A and B
  can not reach B.(B is the first station that A can not reach.)
• If the total number of gas is bigger than the total number of cost. There must be a solution.

首先判断是否有solution，如果有solution，判断起点在哪里，如果是负数，那么start就是下一个。整个循环是有解的；

class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int sum = 0;
        for(int i = 0; i < gas.length; i++) {
            sum += gas[i] - cost[i];
        }
        if(sum < 0) {
            return -1;
        }
        
        int start = 0;
        sum = 0;
        for(int i = 0; i < gas.length; i++) {
            sum += gas[i] - cost[i];
            if(sum < 0) {
                start = i + 1;
                sum = 0;
            }
        }
        return start;
    }
}