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Java-based LeetCode algorithm problem solutions, regularly updated
package g0101_0200.s0130_surrounded_regions;
// #Medium #Top_Interview_Questions #Array #Depth_First_Search #Breadth_First_Search #Matrix
// #Union_Find #2022_02_22_Time_2_ms_(80.58%)_Space_52_MB_(17.02%)
public class Solution {
public void solve(char[][] board) {
// Edge case, empty grid
if (board.length == 0) {
return;
}
// Traverse first and last rows ( boundaries)
for (int i = 0; i < board[0].length; i++) {
// first row
if (board[0][i] == 'O') {
// It will covert O and all it's touching O's to #
dfs(board, 0, i);
}
// last row
if (board[board.length - 1][i] == 'O') {
// Coverts O's to #'s (same thing as above)
dfs(board, board.length - 1, i);
}
}
// Traverse first and last Column (boundaries)
for (int i = 0; i < board.length; i++) {
// first Column
if (board[i][0] == 'O') {
// Converts O's to #'s
dfs(board, i, 0);
}
// last Column
if (board[i][board[0].length - 1] == 'O') {
// Coverts O's to #'s
dfs(board, i, board[0].length - 1);
}
}
// Traverse through entire matrix
for (int i = 0; i < board.length; i++) {
for (int j = 0; j < board[0].length; j++) {
if (board[i][j] == 'O') {
// Convert O's to X's
board[i][j] = 'X';
}
if (board[i][j] == '#') {
// Convert #'s to O's
board[i][j] = 'O';
}
}
}
}
void dfs(char[][] board, int row, int column) {
if (row < 0
|| row >= board.length
|| column < 0
|| column >= board[0].length
|| board[row][column] != 'O') {
return;
}
board[row][column] = '#';
dfs(board, row + 1, column);
dfs(board, row - 1, column);
dfs(board, row, column + 1);
dfs(board, row, column - 1);
}
}