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
Solution: Traverse from the boarder to the inside and mark all the 'O's that are not surrounded by 'X' as 'V' (visited).
1. DFS.
2. BFS (queue).
1 class Solution { 2 public: 3 void solve(vector<vector<char>> &board) { 4 if (board.empty() || board[0].empty()) return; 5 int N = board.size(), M = board[0].size(); 6 for (int i = 0; i < N; ++i) 7 for (int j = 0; j < M; ++j) 8 if (i == 0 || j == 0 || i == N-1 || j == M-1) 9 dfs(board, i, j); // you may call dfs or bfs here! 10 for (int i = 0; i < N; ++i) 11 for (int j = 0; j < M; ++j) 12 board[i][j] = (board[i][j] == 'V') ? 'O' : 'X'; 13 } 14 15 // runtime error 16 void dfs(vector<vector<char> > &board, int row, int col) { 17 int N = board.size(), M = board[0].size(); 18 if (row < 0 || row >= N || col < 0 || col >= M) return; 19 if (board[row][col] != 'O') return; 20 board[row][col] = 'V'; 21 if(row+1 < N && board[row+1][col] == 'O') dfs(board, row+1, col); 22 if(row-1 >= 0 && board[row-1][col] == 'O') dfs(board, row-1, col); 23 if(col+1 < N && board[row][col+1] == 'O') dfs(board, row, col+1); 24 if(col-1 >= 0 && board[row][col-1] == 'O') dfs(board, row, col-1); 25 } 26 27 void bfs(vector<vector<char> > &board, int row, int col) { 28 if (board[row][col] != 'O') return; 29 int N = board.size(), M = board[0].size(); 30 queue<pair<int, int>> q; 31 q.push(make_pair(row, col)); 32 while (!q.empty()) 33 { 34 int i = q.front().first, j = q.front().second; 35 q.pop(); 36 if (i < 0 || i >= N || j < 0 || j >= M) continue; 37 if (board[i][j] != 'O') continue;// important to recheck! 38 board[i][j] = 'V'; 39 q.push(make_pair(i-1, j)); 40 q.push(make_pair(i+1, j)); 41 q.push(make_pair(i, j-1)); 42 q.push(make_pair(i, j+1)); 43 } 44 } 45 };
