Minimum Time Difference LeetCode solution in C++

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class Solution {
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 public:
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  int findMinDifference(vector<string> &timePoints) {
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    unordered_set<string> st;
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    for (auto &i : timePoints) {
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      if (st.count(i)) return 0;
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      st.insert(i);
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    }
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    int ans = INT_MAX;
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    int first = -1, prev = 0; // first variable will take the diffrence of the
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                              // first time stamp given in the input and 00:00
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    int hour = 0, minute = 0;
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    while (hour < 24) {
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      minute = 0;
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      while (minute < 60) {
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        string hh = to_string(hour), mm = to_string(minute);
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        if (hh.size() == 1) hh = '0' + hh;
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        if (mm.size() == 1) mm = '0' + mm;
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        string p = hh + ":" + mm;
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        if (st.count(p)) {
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          if (first == -1) {
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            first = prev;
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          } else {
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            ans = min(ans, prev);
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          }
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          prev = 0;
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        }
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        prev++;
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        minute++;
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      }
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      hour++;
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    }
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    ans = min(first + prev, ans);
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    return ans;
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  }
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};