leetcode 139: Gas Station

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解决一个车辆能否通过遍历环路加油站完成一圈的问题。该问题涉及计算每个加油站的油量与到达下一个加油站所需的油耗，判断是否能从某个起点出发完成整个环路。

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Gas Station

Total Accepted: 12612 Total Submissions: 51726

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.

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 -1;
        
        int sum = 0;
        int start = 0;
        int totalGas = 0;
        int totalCost = 0;
        for(int i=0; i<gas.length; i++)  {
            sum += gas[i] - cost[i];
            totalGas += gas[i];
            totalCost += cost[i];
            if(sum<0) {
                sum = 0;
                start = i+1;
            } 
        }
        
        return totalGas>=totalCost ? start : -1; 

    }
}