leetcode刷题系列C++-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.

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class Solution {
public:
    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
        const int length = gas.size();
        int sum = 0;
        int total = 0;
        int j = -1;
        for(int i = 0; i < length; ++i)
        {
            sum += gas[i] - cost[i];
            total += gas[i] - cost[i];
            if(sum < 0)
            {
                j = i ;
                sum = 0;
            }
        }
        if(total < 0)
        {
            return -1;
        }
        else
        {
            return j + 1;
        }
    }
};

题目详解：

1.total值是用来记录整个能量圈的正负，如果整个圈都是负值，那么怎么跑都不可能跑完

2.sum值是用来记录里面局部的能量正负，局部可能会gas特别多可以用于下一段，也可能缺失，直接跳到下一个点进行。

总之，只要能量圈的总值是正的，那么就肯定能跑完有解。