【LeetCode】Gas Station

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.

思路：顺序扫一圈。如果总油量小于总消耗，说明起点错误，则将起点往后移位。

几个地方需要注意：

1、如果有解，则终点绕一圈之后回到起点

2、最多绕一圈即可判断是有解还是无解，起点重复代表无解

3、终点必须在起点之后

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        vector<int>::size_type size = gas.size();
        int start = 0;
        int finish = 0;
        int totalGas = gas[start];
        int totalCost = cost[start];
        while(finish != start+size)
        {//if finish == start+size, finish
            if(totalGas >= totalCost)
            {
                finish ++;
                totalGas += gas[finish%size];
                totalCost += cost[finish%size];
            }
            else
            {
                totalGas -= gas[start];
                totalCost -= cost[start];
                start ++;
                if(start == size)
                    return -1;  //no solution
                //restart here
                if(finish+1 == start)
                {
                    finish ++;
                    totalGas = gas[start];
                    totalCost = cost[start];
                }
            }
        }

        return start;
    }
};