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class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int ans = -1;
        int sum = 0;
        for(int i = 0;i < gas.size();i++) {
            sum += gas[i];
            sum -= cost[i];
        }
        if(sum < 0)return ans;

        int now = 0;
        int i = 0;
        ans = 0;
        while(i < gas.size()) {
            now += gas[i] - cost[i];
            if(now < 0) {
                i++;
                now = 0;
                ans = i;
            }
            else
                i++;
        }
        return ans;
    }
};