22. Surrounded Regions

Surrounded Regions

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

思想： 先从边缘是 'O' 的结点开始进行深搜，若是 'O', 则设置已访问为 true. 接着对其余所有非边界结点进行遍历，若是 'O' 且未访问，则将该结点重置为 'X' .

typedef pair<int, int> point;
void dfs(point p, int r, int c, vector<vector<char> > &board, vector<vector<bool> > &visited) {
    visited[p.first][p.second] = true;
    int dx[] = {-1, 0, 1, 0}, dy[] = {0, -1, 0, 1};
    for(int i = 0; i < 4; ++i) {
        int x = p.first + dx[i], y = p.second + dy[i];
        if(0 < x && x < r && 0 < y && y < c && board[x][y] == 'O' && !visited[x][y])
            dfs(point(x, y), r, c, board, visited);
    }
}
class Solution {
public:
	void solve(vector<vector<char>> &board) {
	    if(!board.size() || !board[0].size()) return;
		int row = board.size(), col = board[0].size();
		vector<vector<bool>> visited(row, vector<bool>(col, false));
		for(int j = 0; j < col; ++j) {
			if(board[0][j] == 'O' && !visited[0][j]) dfs(point(0, j), row, col, board, visited);
			if(board[row-1][j] == 'O' && !visited[row-1][j]) dfs(point(row-1, j), row, col, board, visited);
		}
		for(int i = 0; i < row; ++i) {
			if(board[i][0] == 'O' && !visited[i][0]) dfs(point(i, 0), row, col, board, visited);
			if(board[i][row-1] == 'O' && !visited[i][row-1]) dfs(point(i, col-1), row, col, board, visited);
		}
		for(int i = 1; i < row-1; ++i) 
			for(int j = 1; j < col-1; ++j) 
				if(board[i][j] == 'O' && !visited[i][j])
					board[i][j] = 'X';
	}
};