[LeetCode]Gas Station

题目描述：(链接)

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.

解题思路：

如果所有花费的汽油之和>所有站点可提供的汽油之和，无解， return -1，这里用gap变量记录该差值，并且用sum变量记录当前差值，如果差值小于0，将当前站点设为index，sum设为0。

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