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 start = gas.size();
         if(start<=0) return -1;
         int cur=0;
         int left = 0;
         while(cur!=start){
             if( gas[cur] +left >= cost[cur]){
                 left+= gas[cur]- cost[cur];
                 ++cur;
             }else{
                 --start;
                 left+= gas[start]- cost[start];
             }
         }
         if(left < 0) return -1;
         return start%gas.size();
    }
};