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.

 1     int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
 2         int curr_gas = 0;
 3         int idx = -1;
 4         int tmp_gas = 0;
 5         for(int i = 0; i < gas.size(); i ++) {
 6             curr_gas += gas[i] - cost[i];
 7             if( idx != -1)
 8                 tmp_gas += gas[i] - cost[i];
 9             
10             if(tmp_gas < 0) {
11                 idx = -1;
12                 tmp_gas = 0;   
13             }
14             
15             if(gas[i] - cost[i] >= 0 && idx == -1) {
16                 idx = i;
17                 tmp_gas += gas[i] - cost[i];
18             }
19         }
20         
21         if(curr_gas >= 0)
22             return idx;
23         else 
24             return -1;
25     }