Decode the Slanted Ciphertext LeetCode solution in C++

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class Solution {
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 public:
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  string decodeCiphertext(string encodedText, int rows) {
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    int n = encodedText.size();
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    // Determining the number of columns
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    int cols = n / rows;
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    vector<vector<char>> mat(rows, vector<char>(cols, ' '));
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    int i = 0, j = 0;
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    int k = 0;
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    string ans = "";
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    // Filling the matrix using encodedText
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    // Row wise
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    for (int i = 0; i < rows; i++) {
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      for (int j = 0; j < cols; j++) {
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        mat[i][j] = encodedText[k++];
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      }
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    }
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    // Only the upper triangular part of the matrix will
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    // contain characters of the originalText
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    // so, this loop traverses that area
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    for (int k = 0; k < n - (rows * (rows - 1)) / 2; k++) {
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      // i, j are the two pointers for tracking rows and columns
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      ans.push_back(mat[i++][j++]);
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      // If any boundary is hit, then column pointer is subtracted
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      // by row_pointer - 1
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      // and row pointer is reset to 0
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      if (i == rows || j == cols) {
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        j -= (i - 1);
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        i = 0;
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      }
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    }
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    // Removing all trailing spaces
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    while (ans.back() == ' ') ans.pop_back();
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    return ans;
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  }
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};