Surrounded Regions Leetcode Solution

In this post, we are going to solve the Surrounded Regions Leetcode Solution problem of Leetcode. This Leetcode problem is done in many programming languages like C++, Java, and Python.

Table of Contents

Problem

Given an m x n matrix board containing 'X' and 'O', capture all regions that are 4-directionally surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region.

Example 1:

Input: board = [["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]]
Output: [["X","X","X","X"],["X","X","X","X"],["X","X","X","X"],["X","O","X","X"]]
Explanation: Notice that an 'O' should not be flipped if:
- It is on the border, or
- It is adjacent to an 'O' that should not be flipped.
The bottom 'O' is on the border, so it is not flipped.
The other three 'O' form a surrounded region, so they are flipped.

Example 2:

Input: board = [["X"]]
Output: [["X"]]

Constraints:

m == board.length
n == board[i].length
1 <= m, n <= 200

board[i][j] is 'X' or 'O'.

Now, let’s see the leetcode solution of Surrounded Regions Leetcode Solution.

Surrounded Regions Leetcode Solution in Python

class Solution:
  def solve(self, board: List[List[str]]) -> None:
    if not board:
      return

    m = len(board)
    n = len(board[0])
    dirs = [0, 1, 0, -1, 0]
    q = deque()

    for i in range(m):
      for j in range(n):
        if i * j == 0 or i == m - 1 or j == n - 1:
          if board[i][j] == 'O':
            q.append((i, j))
            board[i][j] = '*'

    # Mark grids that stretch from four sides with '*'
    while q:
      i, j = q.popleft()
      for k in range(4):
        x = i + dirs[k]
        y = j + dirs[k + 1]
        if x < 0 or x == m or y < 0 or y == n:
          continue
        if board[x][y] != 'O':
          continue
        q.append((x, y))
        board[x][y] = '*'

    for row in board:
      for i, c in enumerate(row):
        row[i] = 'O' if c == '*' else 'X'

Surrounded Regions Leetcode Solution in CPP

class Solution {
 public:
  void solve(vector<vector<char>>& board) {
    if (board.empty())
      return;

    const int m = board.size();
    const int n = board[0].size();
    const vector<int> dirs{0, 1, 0, -1, 0};
    queue<pair<int, int>> q;

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (i * j == 0 || i == m - 1 || j == n - 1)
          if (board[i][j] == 'O') {
            q.emplace(i, j);
            board[i][j] = '*';
          }

    // Mark grids that stretch from four sides with '*'
    while (!q.empty()) {
      const auto [i, j] = q.front();
      q.pop();
      for (int k = 0; k < 4; ++k) {
        const int x = i + dirs[k];
        const int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (board[x][y] != 'O')
          continue;
        q.emplace(x, y);
        board[x][y] = '*';
      }
    }

    for (vector<char>& row : board)
      for (char& c : row)
        if (c == '*')
          c = 'O';
        else if (c == 'O')
          c = 'X';
  }
};

Surrounded Regions Leetcode Solution in Java

class Solution {
  public void solve(char[][] board) {
    if (board.length == 0)
      return;

    final int m = board.length;
    final int n = board[0].length;
    final int[] dirs = {0, 1, 0, -1, 0};
    Queue<int[]> q = new ArrayDeque<>();

    for (int i = 0; i < m; ++i)
      for (int j = 0; j < n; ++j)
        if (i * j == 0 || i == m - 1 || j == n - 1)
          if (board[i][j] == 'O') {
            q.offer(new int[] {i, j});
            board[i][j] = '*';
          }

    // Mark grids that stretch from four sides with '*'
    while (!q.isEmpty()) {
      final int i = q.peek()[0];
      final int j = q.poll()[1];
      for (int k = 0; k < 4; ++k) {
        final int x = i + dirs[k];
        final int y = j + dirs[k + 1];
        if (x < 0 || x == m || y < 0 || y == n)
          continue;
        if (board[x][y] != 'O')
          continue;
        q.offer(new int[] {x, y});
        board[x][y] = '*';
      }
    }

    for (char[] row : board)
      for (int i = 0; i < row.length; ++i)
        if (row[i] == '*')
          row[i] = 'O';
        else if (row[i] == 'O')
          row[i] = 'X';
  }
}

Note: This problem Surrounded Regions is generated by Leetcode but the solution is provided by Chase2learn This tutorial is only for Educational and Learning purpose.