200. Number of Islands
BFS
implementation
```cpp
int numIslands(vector<vector>& grid) {
int dr[] = {-1, 1, 0, 0};
int dc[] = {0, 0, -1, 1};
queue<pair<int, int>> qu;
int n = grid.size();
int m = grid[0].size();
vector<vector> used(n, vector(m, false));
int count = 0;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (grid[i][j] == '1' and !used[i][j]) {
count ++;
qu.push({i, j});
used[i][j] = true;
while (!qu.empty()) {
auto [r, c] = qu.front(); qu.pop();
for (int d = 0; d < 4; d++) {
int rr = r + dr[d];
int cc = c + dc[d];
if (rr < 0 or rr >= n or cc < 0 or cc >= m) continue;
if (used[rr][cc]) continue;
if (grid[rr][cc] == '0') continue;
qu.push({rr, cc});
used[rr][cc] = true;
}
}
}
}
}
return count;
}
```
</details>
## DFS
## DSU