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     }