leetcode17gas-station

题目描述

环形路上有n个加油站，第i个加油站的汽油量是gas[i].

你有一辆车，车的油箱可以无限装汽油。从加油站i走到下一个加油站（i+1）花费的油量是cost[i]，你从一个加油站出发，刚开始的时候油箱里面没有汽油。

求从哪个加油站出发可以在环形路上走一圈。返回加油站的下标，如果没有答案的话返回-1。

注意：

答案保证唯一。

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.

输入

[2,3,1],[3,1,2]

输出

1

class Solution {
public:
    /**
     * 
     * @param gas int整型vector 
     * @param cost int整型vector 
     * @return int整型
     */
    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
        // write code here
        int  start=gas.size()-1;
        int end=0;
        int sum=gas[start]-cost[start];
        while (start>end)
        {
            if (sum>=0){
                sum+=gas[end]-cost[end];
                ++end;
                
            }else {
                --start;
                sum+=gas[start]-cost[start];
            }
        }
        return sum>=0 ?start:-1;
        
    }
};