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class Solution {
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  boolean isClosed = true;
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  public void solve(char[][] board) {
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    int m = board.length;
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    int n = board[0].length;
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    // To identify all those O which are adjacent and unbounded by 'X', we put a temporary value
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    for (int i = 0; i < m; i++) {
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      for (int j = 0; j < n; j++) {
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        if (board[i][j] == 'O' && (i == 0 || j == 0 || i == m - 1 || j == n - 1)) {
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          dfs(board, i, j);
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        }
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      }
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    }
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    // revert the temperoray value and also replace remaining O with X
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    for (int i = 0; i < m; i++) {
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      for (int j = 0; j < n; j++) {
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        if (board[i][j] == 'T') board[i][j] = 'O';
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        else if (board[i][j] == 'O') board[i][j] = 'X';
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      }
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    }
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  }
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  public void dfs(char[][] board, int i, int j) {
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    if (i < 0 || j < 0 || i >= board.length || j >= board[0].length || board[i][j] != 'O') return;
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    board[i][j] = 'T'; // to put a temperory mark/ to mark as visited
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    dfs(board, i, j + 1); // Top
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    dfs(board, i, j - 1); // Bottom
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    dfs(board, i + 1, j); // Right
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    dfs(board, i - 1, j); // Left
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  }
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}