[leetcode]Gas Station

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

解题思路：先计算所有的代价和即gas[i] - cost[i]如果其和大于等于0说明有解，如果小于0说明无解。

然后再统计从第一个点起的累加如果当其和小于0时说明此点到当前结点都不适合作为开始点则从下一点开始计数

有点类似最大子数组求和中的一部分

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