[leetcode]Gas Station

Gas Station

 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.

算法思路：

先找到哪些加油站可以安全起步，然后再逐个从这些安全的加油站出发。

代码如下：

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int[] stations = new int[gas.length];
        int count = gas.length,index = 0;
        for(int i = 0; i < count;i++){
            if(gas[i] >= cost[i]){
                stations[index++] = i;
            }
        }
        breakable:
        for(int i = 0; i < index; i++){
            int gasCount = 0;
            int costCount = 0;
            for(int j = stations[i]; j < count + stations[i]; j++){
                gasCount += gas[j % count];
                costCount += cost[j % count];
                if(costCount > gasCount) continue breakable;
            }
            return stations[i];
        }
        return -1;
    }
}

 稍稍优化一下下：

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int count = gas.length;
        for(int i = 0; i < count; i++){
            if(gas[i] >= cost[i]){
                int totalCost = cost[i];
                int totalGas = gas[i];
                int get = i;
                while(totalGas >= totalCost){
                    get = (get + 1) % count;
                    if(get == i) return i;
                    totalCost += cost[get];
                    totalGas += gas[get];
                }
            }
        }
        return -1;
    }
}