/* Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.
A region is captured by flipping all 'O's into 'X's in that surrounded region .
For example,
X X X X
X O O X
X X O X
X O X X
After running your function, the board should be:
X X X X
X X X X
X X X X
X O X X
*/
public class Solution {
// This algorithm cannot solve the large test set
public void solve(char[][] board) {
// Start typing your Java solution below
// DO NOT write main() function
int rows = board. length;
if(rows == 0) return;
int cols = board[0].length;
for(int i = 0; i < cols; i++) {
// check first row's O
if(board[0][i] == 'O') {
// change it to other symbol
board[0][i] = '#';
dfs(board, 0, i);
}
// check the last row
if(board[rows - 1][i] == 'O') {
board[rows - 1][i] = '#';
dfs(board, rows - 1, i);
}
}
for(int i = 0; i < rows; i++) {
// check first col
if(board[i][0] == 'O') {
board[i][0] = '#';
dfs(board, i, 0);
}
// check last col
if(board[i][cols - 1] == 'O') {
board[i][cols - 1] = '#';
dfs(board, i, cols - 1);
}
}
// change O to X
changeTo(board, 'O', 'X');
// change # to O
changeTo(board, '#', 'O');
return;
}
public void dfs(char[][] board, int row, int col) {
// check up
if(row > 0) {
if(board[row - 1][col] == 'O') {
board[row - 1][col] = '#';
dfs(board, row - 1, col);
}
}
// check left
if(col > 0) {
if(board[row][col - 1] == 'O') {
board[row][col - 1] = '#';
dfs(board, row, col - 1);
}
}
// check right
if(row < board. length - 1) {
if(board[row + 1][col] == 'O') {
board[row+1][col] = '#';
dfs(board, row+1, col);
}
}
// check down
if(col < board[0].length - 1) {
if(board[row][col+1] == 'O'){
board[row][col+1] = '#';
dfs(board, row, col+1);
}
}
return;
}
public void changeTo(char[][] board, char from, char to) {
for(int i = 0; i < board. length; i++) {
for(int j = 0; j < board[0].length; j++) {
if(board[i][j] == from) {
board[i][j] = to;
}
}
}
return;
}
}