Gas Station [LeetCode]

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.

Solution:

The brute force solution costs O(n^2), I find a O(n) and one round solution. If gas[i] >= cost[i], then I mark it, and calcuate the gas in tank along next node, if it is less than zero, then restart search.

     int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
         int curr_gas = ;
         int idx = -;
         int tmp_gas = ;
         for(int i = ; i < gas.size(); i ++) {
             curr_gas += gas[i] - cost[i];
             if( idx != -)
                 tmp_gas += gas[i] - cost[i];

             if(tmp_gas < ) {
                 idx = -;
                 tmp_gas = ;
             }

             if(gas[i] - cost[i] >=  && idx == -) {
                 idx = i;
                 tmp_gas += gas[i] - cost[i];
             }
         }

         if(curr_gas >= )
             return idx;
         else
             return -;
     }