Gas Station LeetCode solution in C++

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class Solution {
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 public:
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  int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
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    int n = gas.size();
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    int start = -1;
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    int sum = 0, gastillnow = 0;
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    for (int i = 0; i < 2 * n; i++) {
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      if (start == i % n) {
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        return i % n;
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      }
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      if (gas[i % n] + gastillnow >= cost[i % n]) { // we can start from this index
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        if (start == -1) start = i;
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        gastillnow += gas[i % n] - cost[i % n];
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      } else if (gastillnow + gas[i % n] < cost[i % n]) { // previous start index was wrong we have
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                                                          // to start from another
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        start = -1;
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        gastillnow = 0;
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      }
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    }
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    return -1;
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  }
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};