[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.

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int total = 0, sum = 0, start = 0;
        for(int i = 0; i < gas.size(); i++)
        {
            total += gas[i] - cost[i];
            sum += gas[i] - cost[i];
            if(sum < 0){
                sum = 0;
                start = i + 1;
            }
        }
        return total < 0 ? -1 : start;
    }
};

对于一个循环数组，如果这个数组整体和sum>=0，那么必然可以再数组中找到这么一个元素，从这个元素出发，绕数组一圈，能报中国累加和一直是非负。

参考：http://www.cnblogs.com/felixfang/p/3814463.html