https://leetcode.com/problems/gas-station/ | my solution in c++

gasstation.cpp

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int st=0, left=0, sum=0; // left-gas left in the tank
        for (int i=0; i<gas.size(); i++) {
            left+=gas[i]-cost[i];
            sum+=gas[i]-cost[i];
            if (sum<0) {
                sum=0;
                st=i+1;
            }
        }
        return left<0 ? -1:st;
    }
};