56. Number of Islands

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        m, n = len(grid), len(grid[0])
        def sink(r, c):                         # flood-fill one island
            stack = [(r, c)]
            grid[r][c] = '0'
            while stack:
                r, c = stack.pop()
                for dr, dc in ((1, 0), (-1, 0), (0, 1), (0, -1)):
                    nr, nc = r + dr, c + dc
                    if 0 <= nr < m and 0 <= nc < n and grid[nr][nc] == '1':
                        grid[nr][nc] = '0'
                        stack.append((nr, nc))
        count = 0
        for r in range(m):
            for c in range(n):
                if grid[r][c] == '1':
                    count += 1
                    sink(r, c)
        return count

class Solution {
    public int numIslands(char[][] grid) {
        int count = 0;
        for (int r = 0; r < grid.length; r++)
            for (int c = 0; c < grid[0].length; c++)
                if (grid[r][c] == '1') { count++; sink(grid, r, c); }
        return count;
    }
    private void sink(char[][] g, int r, int c) {     // DFS flood fill
        if (r < 0 || c < 0 || r >= g.length || c >= g[0].length || g[r][c] != '1') return;
        g[r][c] = '0';
        sink(g, r + 1, c); sink(g, r - 1, c); sink(g, r, c + 1); sink(g, r, c - 1);
    }
}

class Solution {
    void sink(vector<vector<char>>& g, int r, int c) {   // DFS flood fill
        if (r < 0 || c < 0 || r >= (int)g.size() || c >= (int)g[0].size() || g[r][c] != '1') return;
        g[r][c] = '0';
        sink(g, r + 1, c); sink(g, r - 1, c); sink(g, r, c + 1); sink(g, r, c - 1);
    }
public:
    int numIslands(vector<vector<char>>& grid) {
        int count = 0;
        for (int r = 0; r < (int)grid.size(); r++)
            for (int c = 0; c < (int)grid[0].size(); c++)
                if (grid[r][c] == '1') { count++; sink(grid, r, c); }
        return count;
    }
};

function numIslands(grid) {
  const m = grid.length, n = grid[0].length;
  const sink = (r, c) => { // DFS flood fill
    if (r < 0 || c < 0 || r >= m || c >= n || grid[r][c] !== '1') return;
    grid[r][c] = '0';
    sink(r + 1, c); sink(r - 1, c); sink(r, c + 1); sink(r, c - 1);
  };
  let count = 0;
  for (let r = 0; r < m; r++)
    for (let c = 0; c < n; c++)
      if (grid[r][c] === '1') { count++; sink(r, c); }
  return count;
}