private static final int[][] DIRS = { { 0, 1 }, { 0, -1 }, { 1, 0 }, { -1, 0 } };

static int n;

static int m;

static char[][] g;

public void solve(char[][] board) {

n = board.length;

m = board[0].length;

g = board;

fillBorders(board, 'O', '*');

for (int i = 0; i < n; i++) {

for (int j = 0; j < m; j++) {

if (board[i][j] == 'O') {

board[i][j] = 'X';

}

}

}

fillBorders(board, '*', 'O');

}

private static void fillBorders(char[][] board, char prev, char next) {

for (int i = 0; i < n; i++) {

if (board[i][0] == prev) { dfs(i, 0, prev, next); }

if (board[i][m - 1] == prev) { dfs(i, m - 1, prev, next); }

}

for (int j = 0; j < m; j++) {

if (board[0][j] == prev) { dfs(0, j, prev, next); }

if (board[n - 1][j] == prev) { dfs(n - 1, j, prev, next); }

}

}

private static void dfs(int x, int y, char prev, char next) {

g[x][y] = next;

for (int[] dir : DIRS) {

final int nx = x + dir[0];

final int ny = y + dir[1];

if (0 <= nx && nx < n && 0 <= ny && ny < m && g[nx][ny] == prev) {

dfs(nx, ny, prev, next);

}

}

}