N-Queens LeetCode solution in C++

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class Solution {
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  bool isSafe(vector<string> board, int row, int col, int n) {
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    int r = row;
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    int c = col;
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    // Checking for upper left diagonal
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    while (row >= 0 && col >= 0) {
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      if (board[row][col] == 'Q') return false;
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      row--;
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      col--;
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    }
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    row = r;
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    col = c;
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    // Checking for left
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    while (col >= 0) {
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      if (board[row][col] == 'Q') return false;
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      col--;
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    }
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    row = r;
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    col = c;
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    // Checking for lower left diagonal
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    while (row < n && col >= 0) {
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      if (board[row][col] == 'Q') return false;
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      row++;
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      col--;
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    }
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    return true;
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  }
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  void solve(vector<vector<string>> &ans, vector<string> &board, int n, int col) {
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    if (col == n) {
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      ans.push_back(board);
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      return;
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    }
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    for (int row = 0; row < n; row++) {
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      if (isSafe(board, row, col, n)) {
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        board[row][col] = 'Q';
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        solve(ans, board, n, col + 1);
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        board[row][col] = '.';
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      }
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    }
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  }
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 public:
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  vector<vector<string>> solveNQueens(int n) {
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    vector<vector<string>> ans;
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    vector<string> board;
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    string s(n, '.');
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    for (int i = 0; i < n; i++) board.push_back(s);
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    solve(ans, board, n, 0);
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    return ans;
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  }
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};