Leetcode Gas Station

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本文探讨了一种特殊的环路旅行问题：给定一系列加油站及其油量，以及从一个站到下一个站所需的油耗，如何确定能否完成一次完整的环路旅行。通过巧妙地利用贪心算法，我们提出了一种有效的方法来解决这个问题。

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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.

Difficulty: Medium

Solution: tricky greedy problem

To solve this problem, we need to understand and use the following 2 facts:
1) if the sum of gas >= the sum of cost, then the circle can be completed.
2) if A can not reach C in a the sequence of A-->B-->C, then B can not make it either.

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int len = gas.length;
        int ans = 0, remaining = 0, totalRemaining = 0;
        for(int i = 0; i < len; i++){
            totalRemaining += gas[i] - cost[i];
            if(remaining < 0){
                ans = i;
                remaining = gas[i] - cost[i];
            }
            else{
                remaining += gas[i] - cost[i];
            }
        }
        if(remaining < 0 || totalRemaining < 0) return -1;
        return ans;
    }
}