find num of island using graph search

0200_number-of-islands.cs

// O(row*col), O(1), O(row+col) stack space

public class Solution {
  char[][] grid;
  
  public int NumIslands(char[,] grid) {
    int count = 0;
    for (int r=0; r<numRows; r++)
      for (int c=0; c<numCols; c++)
        if (grid[r][c] == '1') {
          dfs(r, c);
          count++;
        }
    return count;
  }
  
  void dfs(int r, int c) {
    if (r < 0 || c < 0 || r >= numRows || c >= numCols || grid[r][c] != '1')
      return ;
    
    grid[r][c] = '0';
    dfs(r-1, c);
    dfs(r, c-1);
    dfs(r, c+1);
    dfs(r+1, c);
  }
}