Surrounded Regions

X X X X
X O O X
X X O X
X O X X

X X X X
X X X X
X X X X
X O X X

 1 class Solution {
 2 public:
 3     void solve( vector<vector<char>> &board ) {
 4         if( board.size() <= 2 || board[0].size() <= 2 ) { return; }
 5         int rows = board.size(), cols = board[0].size();
 6         vector<vector<char>> flip_board( rows, vector<char>( cols, ‘#‘ ) );
 7         list<pair<int,int>> candidates;
 8         // 将左、右边界上的‘O‘加入candidates队列。
 9         for( int i = 0; i < rows; ++i ) {
10             flip_board[i][0] = board[i][0];
11             flip_board[i][cols-1] = board[i][cols-1];
12             if( board[i][0] == ‘O‘ ) { candidates.push_back( pair<int,int>(i,0) ); }
13             if( board[i][cols-1] == ‘O‘ ) { candidates.push_back( pair<int,int>(i,cols-1) ); }
14         }
15         // 将上、下边界上的‘O‘加入candidates队列。
16         for( int j = 1; j < cols-1; ++j ) {
17             flip_board[0][j] = board[0][j];
18             flip_board[rows-1][j] = board[rows-1][j];
19             if( board[0][j] == ‘O‘ ) { candidates.push_back( pair<int,int>(0,j) ); }
20             if( board[rows-1][j] == ‘O‘ ) { candidates.push_back( pair<int,int>(rows-1,j) ); }
21         }
22         // 取队首元素，对其上、下、左、右侧的邻居进行扩展。扩展条件为：该邻居未被访问过，即flip_board[i][j]为‘#‘，且该邻居取值为‘O‘。
23         while( !candidates.empty() ) {
24             int x = candidates.front().first, y = candidates.front().second;
25             candidates.pop_front();
26             if( x-1 >= 0 && flip_board[x-1][y] == ‘#‘ ) {
27                 flip_board[x-1][y] = board[x-1][y];
28                 if( board[x-1][y] == ‘O‘ ) { candidates.push_back( pair<int,int>(x-1,y) ); }
29             }
30             if( x+1 < rows && flip_board[x+1][y] == ‘#‘ ) {
31                 flip_board[x+1][y] = board[x+1][y];
32                 if( board[x+1][y] == ‘O‘ ) { candidates.push_back( pair<int,int>(x+1,y) ); }
33             }
34             if( y-1 >= 0 && flip_board[x][y-1] == ‘#‘ ) {
35                 flip_board[x][y-1] = board[x][y-1];
36                 if( board[x][y-1] == ‘O‘ ) { candidates.push_back( pair<int,int>(x,y-1) ); }
37             }
38             if( y+1 < cols && flip_board[x][y+1] == ‘#‘ ) {
39                 flip_board[x][y+1] = board[x][y+1];
40                 if( board[x][y+1] == ‘O‘ ) { candidates.push_back( pair<int,int>(x,y+1) ); }
41             }
42         }
43         for( int i = 0; i < rows; ++i ) {
44             for( int j = 0; j < cols; ++j ) {
45                 if( flip_board[i][j] == ‘#‘ ) { flip_board[i][j] = ‘X‘; }
46             }
47         }
48         board = flip_board;
49         return;
50     }
51 };