Leetcode 134 gas-station

题目：

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

You have a car with an unlimited gas tank and it costscost[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.

思路：

从start出发， 如果油量足够， 可以一直向后走 end++；  油量不够的时候，


start向后退  最终 start == end的时候，如果有解一定是当前 start所在位置

代码：

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int end = 0;
        int start = cost.length-1;
        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;
          
    }
}