<p>You are given an <code>m x n</code> matrix <code>board</code> containing <strong>letters</strong> <code>&#39;X&#39;</code> and <code>&#39;O&#39;</code>, <strong>capture regions</strong> that are <strong>surrounded</strong>:</p>

<ul>
	<li><strong>Connect</strong>: A cell is connected to adjacent cells horizontally or vertically.</li>
	<li><strong>Region</strong>: To form a region <strong>connect every</strong> <code>&#39;O&#39;</code> cell.</li>
	<li><strong>Surround</strong>: The region is surrounded with <code>&#39;X&#39;</code> cells if you can <strong>connect the region </strong>with <code>&#39;X&#39;</code> cells and none of the region cells are on the edge of the <code>board</code>.</li>
</ul>

<p>A <strong>surrounded region is captured</strong> by replacing all <code>&#39;O&#39;</code>s with <code>&#39;X&#39;</code>s in the input matrix <code>board</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">board = [[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;O&quot;,&quot;O&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;X&quot;,&quot;O&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;O&quot;,&quot;X&quot;,&quot;X&quot;]]</span></p>

<p><strong>Output:</strong> <span class="example-io">[[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;O&quot;,&quot;X&quot;,&quot;X&quot;]]</span></p>

<p><strong>Explanation:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/xogrid.jpg" style="width: 367px; height: 158px;" />
<p>In the above diagram, the bottom region is not captured because it is on the edge of the board and cannot be surrounded.</p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">board = [[&quot;X&quot;]]</span></p>

<p><strong>Output:</strong> <span class="example-io">[[&quot;X&quot;]]</span></p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>m == board.length</code></li>
	<li><code>n == board[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 200</code></li>
	<li><code>board[i][j]</code> is <code>&#39;X&#39;</code> or <code>&#39;O&#39;</code>.</li>
</ul>

class Solution {
public:
    int m1,n1;
    vector<int> dx,dy;
    bool isBdry(int &x,int &y){
        return (x==0 || y==0 || x == m1 || y == n1);
    }
    bool isValid(int &x,int &y){
        return (x>=0 && y>=0 && x<=m1 && y<=n1);
    }
    bool isCaptured(int i,int j,vector<vector<char>>& board){
        board[i][j] = 'N';
        bool result = true;
        for(int k=0,x,y;k<4;k++){
            x = i + dx[k];
            y = j + dy[k];
            if(!isValid(x,y) || board[x][y] != 'O')
                continue;
            if(isBdry(x,y))
                result = false;
            if(!isCaptured(x,y,board))
                result = false;
        }
        return result;
    }
    void setValue(int i,int j,char &c,vector<vector<char>>& board){
        board[i][j] = c;
        for(int k=0,x,y;k<4;k++){
            x = i + dx[k];
            y = j + dy[k];
            if(isValid(x,y) && board[x][y] == 'N'){
                board[x][y] = c;
                setValue(x,y,c,board);
            }
        }
    }
    void solve(vector<vector<char>>& board) {
        m1 = board.size() - 1;
        n1 = board[0].size() - 1;
        dx = {1,-1,0,0};
        dy = {0,0,1,-1};
        char X ='X' , N = 'N';
        for(int i=1;i<m1;i++)
        for(int j=1;j<n1;j++)
        if(board[i][j] == 'O'){
            if(isCaptured(i,j,board))
                setValue(i,j,X,board);
        }
        for(int i=0;i<=m1;i++)
        for(int j=0;j<=n1;j++)
            if(board[i][j] ==N)
                board[i][j] = 'O';

    }
};
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