【LeetCode】Gas Station 解題報告

【題目】

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.

【題意】

環形路線上有N個加油站，每個加油站有汽油gas[i]，從每個加油站到下一站消耗汽油cost[i]，問從哪個加油站出發能夠回到起始點，如果都不能則返回-1（注意，解是唯一的）。

【Java代碼】（O(n^2)）

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        //依次從每一個加油站出發
        for (int i = 0; i < gas.length; i++) {
            int j = i;
            int curgas = gas[j];

            while (curgas >= cost[j]) { //如果當前的汽油量能夠到達下一站
                curgas -= cost[j];      //減去本次的消耗

                j = (j + 1) % gas.length; //到達下一站
                if (j == i) return i;   //如果回到了起始站，那麼旅行成功

                curgas += gas[j];       //到下一站後重新加油，繼續前進
            }
        }
        return -1;
    }
}

【Java代碼】（O(n)）

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int sum = 0;
        int total = 0;
        int j = -1;
        for (int i = 0; i < gas.length; i++) {
            sum += gas[i] - cost[i];
            total += gas[i] - cost[i];
            if(sum < 0) {   //之前的油量不夠到達當前加油站
                j = i;
                sum = 0;
            }
        }
        if (total < 0) return -1;    //所有加油站的油量都不夠整個路程的消耗
        else return j + 1;
    }
}