LeetCode 134 Gas Station

题目描述

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.

分析

如果能从i开始完成一圈，那么：

sum(gas[0..end])>=sum(cost[0..end])sum(gas[0..end])>=sum(cost[0..end])

sum(gas[0..i−1])<=sum(cost[0..i−1])sum(gas[0..i-1])<=sum(cost[0..i-1])

代码

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

        int start = 0;
        int from_start = 0;

        int total = 0;

        for (int i = 0; i < cost.length; i++) {
            int left = gas[i] - cost[i];
            total += left;
            from_start += left;

            if (from_start < 0) {
                from_start = 0;
                start = i + 1;
            }
        }

        if (total >= 0) {
            return start;
        }

        return -1;
    }