Kth Smallest Element in a Sorted Matrix LeetCode solution in C++

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class Solution {
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 public:
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  int kthSmallest(vector<vector<int>> &matrix, int k) {
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    int n = matrix.size(), start = matrix[0][0], end = matrix[n - 1][n - 1];
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    unordered_map<int, int> mp;
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    for (int i = 0; i < n; i++)
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      for (int j = 0; j < n; j++) mp[matrix[i][j]]++;
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    for (int i = start; i <= end; i++)
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      if (mp.find(i) != mp.end()) {
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        for (int j = 0; j < mp[i]; j++) {
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          k--;
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          if (k == 0) return i;
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        }
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      }
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    return -1;
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  }
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};