Middle-题目82:134. Gas Station
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题目原文:
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.
题目大意:
有n个加油站围成一个圈,其中第i个加油站的油量是gas[i].
你有一个汽车,每从第i个加油站开到第i+1个,要消耗cost[i]的汽油。初始油箱为空,问这个汽车能否开完一圈,如果能则返回一个起点位置,否则返回-1.
题目分析:
首先朴素算法就是从每个节点开始开一圈,看看能不能走到头。这样提交会超时。
那么从discuss看到的一个贪心算法,从节点0开始向前开,如果开完一个站点还有油就向前开,如果油量不足,就把出发站点往回移动一站,并从出发点重新开始开,直到遍历完一圈,如果最后油箱里的油≥0,则返回最后的出发点,否则返回-1.
源码:(language:java)
public class Solution { public int canCompleteCircuit(int[] gas, int[] cost) { if (gas == null) { return -1; } // Note: The Solution object is instantiated only once and is reused by each test case. int count = gas.length; int n = 0; int gasInCar = 0; int begin = 0; int end = 0; int i = 0; while (n < count - 1) { gasInCar += gas[i] - cost[i]; if (gasInCar >=0) {//forward end++; i=end; } else { begin--; if (begin < 0) { begin = count - 1; } i = begin; } n++; } gasInCar += gas[i] - cost[i]; if (gasInCar >= 0) { return begin; } else { return -1; } }}
成绩:
1ms,beats 5.79%,众数1ms,82.52%
Cmershen的碎碎念:
这个算法的理解和实现都不算难,但暂时没想到如何严格证明其正确性。
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