1 class Solution {
2 public:
3 void solve(vector<vector<char>>& board) {
4 if (board.empty()) return;
5 int n = board.size();
6 int m = board[0].size();
7 // Top-most line
8 for (int j = 0; j < m; ++j)
9 if (board[0][j] == 'O')
10 DFS(board, 0, j);
11 // Right-most column
12 for (int i = 1; i < n; ++i)
13 if (board[i][m-1] == 'O')
14 DFS(board, i, m - 1);
15 // Bottom-most line
16 for (int j = 0; j < m - 1; ++j)
17 if (board[n-1][j] == 'O')
18 DFS(board, n - 1, j);
19 // Left-most column
20 for (int i = 1; i < n - 1; ++i)
21 if (board[i][0] == 'O')
22 DFS(board, i, 0);
23 // Change interior 'O's to 'X's meanwhile restoring '#'s to 'O's
24 for (auto& vec: board)
25 for (char& c : vec)
26 if (c == 'O')
27 c = 'X';
28 else if (c == '#')
29 c = 'O';
30 }
31
32 void DFS(vector<vector<char>>& board, int i, int j) {
33 board[i][j] = '#';
34 if (i - 1 >= 0 && board[i-1][j] == 'O')
35 DFS(board, i - 1, j);
36 if (j + 1 < board[0].size() && board[i][j+1] == 'O')
37 DFS(board, i, j + 1);
38 if (i + 1 < board.size() && board[i+1][j] == 'O')
39 DFS(board, i + 1, j);
40 if (j - 1 >= 0 && board[i][j-1] == 'O')
41 DFS(board, i, j - 1);
42 }
43
44 void BFS(vector<vector<char>>& board, int row, int col) {
45 board[row][col] = '#';
46 queue<pair<int, int>> Q;
47 Q.push(make_pair(row, col));
48 while (!Q.empty()) {
49 int i = Q.front().first;
50 int j = Q.front().second;
51 Q.pop();
52 if (i - 1 >= 0 && board[i-1][j] == 'O') {
53 board[i-1][j] = '#';
54 Q.push(make_pair(i - 1, j));
55 }
56 if (j + 1 < board[0].size() && board[i][j+1] == 'O') {
57 board[i][j+1] = '#';
58 Q.push(make_pair(i, j + 1));
59 }
60 if (i + 1 < board.size() && board[i+1][j] == 'O') {
61 board[i+1][j] = '#';
62 Q.push(make_pair(i + 1, j));
63 }
64 if (j - 1 >= 0 && board[i][j-1] == 'O') {
65 board[i][j-1] = '#';
66 Q.push(make_pair(i, j - 1));
67 }
68 }
69 }
70 };
