# [130\. Surrounded Regions](https://leetcode.com/problems/surrounded-regions/)

:::spoiler Solution

class Solution {
public:
    void solve(vector<vector<char>>& board)
    {
        int rows = board.size(), cols = board[0].size();
        for (int i = 0; i < rows; i++)
        {
            for (int j = 0; j < cols; j++)
            {
                if (board[i][j] == 'O' && (i == 0 || j == 0 || i == rows - 1 || j == cols - 1))
                {
                    dfs(board, i, j);
                }
            }
        }
        
        for (int i = 0; i < rows; i++)
        {
            for (int j = 0; j < cols; j++)
            {
                if (board[i][j] == 'O') board[i][j] = 'X';
                if (board[i][j] == '#') board[i][j] = 'O';
            }
        }        
    }

    # a dfs function designed to mark boundary grids with '#' characters
    void dfs(vector<vector<char>>& board, int i, int j)
    {
        int rows = board.size(), cols = board[0].size();
        if(i < 0 || j < 0 || i >= rows || j >= cols || board[i][j] != 'O')
            return;
        
        board[i][j] = '#';
        
        vector<vector<int>> directions = {{0, -1}, {0, 1}, {1, 0}, {-1, 0}};
        
        for(auto dir : directions)
            dfs(board, i + dir[0], j + dir[1]);
    }
};

- T: $O()$
- S: $O()$
:::