134. Gas Station

题目：

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. 

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链接：  http://leetcode.com/problems/gas-station/

题解：

经典题gas station，贪婪法。设计一个局部当前的gas，一个全局的gas，当局部gas小于0时，局部gas置零，设置结果为当前index的下一个位置，此情况可能出现多次。当全局gas小于0的话说明没办法遍历所有gas站，返回-1。

Time Complexity - O(n)， Space Complexity - O(1)。

public class Solution {

    public int canCompleteCircuit(int[] gas, int[] cost) {

        if(gas == null || cost == null || gas.length == 0 || cost.length == 0 || gas.length != cost.length)

            return 0;

        int curGas = 0;

        int totalGas = 0;

        int result = 0;

        

        for(int i = 0; i < gas.length; i++){

            curGas += gas[i] - cost[i];

            totalGas += gas[i] - cost[i];

            if(curGas < 0){

                result = i + 1;

                curGas = 0;

            }

        }

        

        if(totalGas < 0)

            return - 1;

        

        return result;

    }

}

 

测试：