【leetcode】No.134：gas-station

题目描述

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.

思路：

环状的加油站路径如果sum(gas[i])>sum(cost[i])则必定是有解的，基本思路是用两个变量，cur和total分别表示当前的油量和总共会剩余油量，如果在某一点cur小于0则说明前面几个站不可能作为起点，则起始站pos变为i+1，最终遍历完之后检查total是否大于0，是的话起点站就为pos，否则就为-1*。

代码：

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int total = 0,cur = 0, pos = 0;
        for (int i=0; i<gas.length; i++){
            total += gas[i] - cost[i];
            cur += gas[i] - cost[i];
            if (cur < 0){
                cur = 0;
                pos = i + 1;
            }
        }
        return total >= 0 ? pos : -1;
    }
}