leetcode134 加油站

直接用brute force，注意数组的循环遍历怎么访问，这里用do {} while();

每个元素访问两遍，时间复杂度O(N^2),当然可以优化到O(N)。

class Solution {
public:
    int canCompleteCircuit(vector& gas, vector& cost) {
        for (int i = 0; i < gas.size(); i++) {
            if (helper(i, gas, cost))
                return i;
        }
        return -1;
    }
private:
    bool helper(int pos, vector& gas, vector& cost) {
        int index = pos;
        int rest = 0;
        /*此处do...while...环形遍历数组*/
        do{
            rest += gas[index] - cost[index];
            if (rest < 0)
                return false;
            if (index == gas.size()-1)
                index = 0;
            else
                index++;
        }while(index!=pos);
        return true;
    }

};