class Solution { public: int closedIsland(vector<vector<int>>& grid) { const int m = grid.size(); const int n = grid[0].size(); // remove lands connected to edge for (int i = 0; i < m; ++i) for (int j = 0; j < n; ++j) if (i * j == 0 || i == m - 1 || j == n - 1) if (grid[i][j] == 0) dfs(grid, i, j); int ans = 0; // reduce to 200. Number of Islands  for (int i = 0; i < m; ++i) for (int j = 0; j < n; ++j) if (grid[i][j] == 0) { dfs(grid, i, j); ++ans; } return ans; } private: void dfs(vector<vector<int>>& grid, int i, int j) { if (i < 0 || i == grid.size() || j < 0 || j == grid[0].size()) return; if (grid[i][j] == 1) return; grid[i][j] = 1; dfs(grid, i + 1, j); dfs(grid, i - 1, j); dfs(grid, i, j + 1); dfs(grid, i, j - 1); }; };