Solution.java

class Solution {
    private int[] p;
    private int[][] dirs = new int[][]{{0, -1}, {0, 1}, {1, 0}, {-1, 0}};

    public void solve(char[][] board) {
        int m = board.length, n = board[0].length;
        p = new int[m * n + 1];
        for (int i = 0; i < p.length; ++i) {
            p[i] = i;
        }
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (board[i][j] == 'O') {
                    if (i == 0 || j == 0 || i == m - 1 || j == n - 1) {
                        p[find(i * n + j)] = find(m * n);
                    } else {
                        for (int[] e : dirs) {
                            if (board[i + e[0]][j + e[1]] == 'O') {
                                p[find(i * n + j)] = find((i + e[0]) * n + j + e[1]);
                            }
                        }
                    }
                }
            }
        }
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (board[i][j] == 'O' && find(i * n + j) != find(m * n)) {
                    board[i][j] = 'X';
                }
            }
        }
    }

    private int find(int x) {
        if (p[x] != x) {
            p[x] = find(p[x]);
        }
        return p[x];
    }
}