leetCode练习（134）（

题目：Gas Station

难度：MEDIUM

问题描述：

There are N gas stations along a circular route, where the amount of gas at stationi is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from stationi 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：

1.如果gas和大于cost和，必定有解。

2.从i开始，如果到k时，gas和小于cost和，那么解必定不在I和k之间（包含两端）

有了上面两点，题目就很容易解了，代码如下：

public class Solution {    public int canCompleteCircuit(int[] gas, int[] cost) {        int total = 0;        int len = gas.length;        for(int i=0;i<len;i++){        total +=gas[i]-cost[i];        }        if(total<0)return -1;                for(int i=0;i<len;i++){        int rest=0;        int index=i;        int count=0;        while(!(rest<0)&&count<len){        rest +=gas[index]-cost[index];        if(rest<0){        break;        }        index++;        count++;        if(index==len) index=0;        }        if(count==len) return index;        else i=index;        }        return -1;    }}