[LeetCode] 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.

class Solution {
public:
  
int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int num = gas.size();
        int remain = 0;//从第一站开始到本站剩余的gas量
        int start = 0;
        for(int i = 0;i<num;)
        {
           if(cost[i]>gas[i])
           {
              i++;
              continue;
           }
           else
           {
              start = i;//标记开始站
              remain = gas[i]-cost[i];
              i++;
              while(i<num &&gas[i]+remain >= cost[i])
              {
                 remain = (gas[i]+remain - cost[i]);
                 i++;
              }
              if(i == num)
              {
                i = 0;
              }
              else
              {
                start = i+1;
                i = start;
                continue;
              }

              while(gas[i]+remain >= cost[i] && i<start)
              {
                 remain = (gas[i]+remain - cost[i]);
                 i++;
              }
                if(i == start)
              {
                 return start;
              }
              else
              {
                return -1;
              }
           }
        }//end for
        return -1;
    }
};