Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

11110 11010 11000 00000 Answer: 1

Example 2:

11000 11000 00100 00011 Answer: 3

Ideas:

• DFS
• Union Find

Union Find Code:

class UF {
  public int count = 0;
  public int[] id = null;

  public UF(int m, int n, char[][] grid) {
      for(int i = 0; i < m; i++) {
          for(int j = 0; j < n; j++) {
              if(grid[i][j] == '1') count++;
          }
      }
      id = new int[m * n];
      for(int i = 0; i < m * n; i++) {
          id[i] = i;
      }
  }

  public int find(int p) {
      while(p != id[p]) {
          id[p] = id[id[p]];
          p = id[p];
      }
      return p;
  }

  public boolean isConnected(int p, int q) {
      int pRoot = find(p);
      int qRoot = find(q);
      if(pRoot != qRoot) return false;
      else return true;
  }

  public void union(int p, int q) {
      int pRoot = find(p);
      int qRoot = find(q);
      if(pRoot == qRoot) return;
      id[pRoot] = qRoot;
      count--;
  }
}

public int numIslands(char[][] grid) {
    if(grid.length == 0 || grid[0].length == 0) return 0;
    int m = grid.length, n = grid[0].length;
    UF uf = new UF(m , n, grid);

    for(int i = 0; i < m; i++) {
        for(int j = 0; j < n; j++) {
            if(grid[i][j] == '0') continue;
            int p = i * n + j;
            int q;
            if(i > 0 && grid[i - 1][j] == '1') {
                q = p - n;
                uf.union(p, q);
            }
            if(i < m - 1 && grid[i + 1][j] == '1') {
                q = p + n;
                uf.union(p, q);
            }
            if(j > 0 && grid[i][j - 1] == '1') {
                q = p - 1;
                uf.union(p, q);
            }
            if(j < n - 1 && grid[i][j + 1] == '1') {
                q = p + 1;
                uf.union(p, q);
            }
        }
    }
    return uf.count;
}