Count Sub Islands — LeetCode 1905 Python Solution

The problem

You are given two m x n binary matrices grid1 and grid2 containing only 0's (representing water) and 1's (representing land). An island is a group of 1's connected 4-directionally (horizontal or vertical).

Example

Input

grid1 = [[1,1,1,0,0],[0,1,1,1,1],[0,0,0,0,0],[1,0,0,0,0],[1,1,0,1,1]], grid2 = [[1,1,1,0,0],[0,0,1,1,1],[0,1,0,0,0],[1,0,1,1,0],[0,1,0,1,0]]

Output

3

Explanation

In the picture above, the grid on the left is grid1 and the grid on the right is grid2.

Python solution

class Solution:
    def countSubIslands(self, grid1: List[List[int]], grid2: List[List[int]]) -> int:
        def dfs(i: int, j: int) -> int:
            ok = grid1[i][j]
            grid2[i][j] = 0
            for a, b in pairwise(dirs):
                x, y = i + a, j + b
                if 0 <= x < m and 0 <= y < n and grid2[x][y] and not dfs(x, y):
                    ok = 0
            return ok

        m, n = len(grid1), len(grid1[0])
        dirs = (-1, 0, 1, 0, -1)
        return sum(dfs(i, j) for i in range(m) for j in range(n) if grid2[i][j])

Complexity

Pattern: Union-Find

Merge groups and ask whether two things are connected, both in near-constant time. LeetCode 1905. Count Sub Islands is filed here because LeetCode tags it Union Find, which is the vocabulary this hub collects.

The union-find guide has the Python template for the pattern and the 83 LeetCode problems that use it.

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