Solution.py

class UnionFind:
    def __init__(self, n: int):
        self.p = list(range(n))
        self.size = [1] * n

    def find(self, x: int):
        if self.p[x] != x:
            self.p[x] = self.find(self.p[x])
        return self.p[x]

    def union(self, a: int, b: int) -> bool:
        pa, pb = self.find(a - 1), self.find(b - 1)
        if pa == pb:
            return False
        if self.size[pa] > self.size[pb]:
            self.p[pb] = pa
            self.size[pa] += self.size[pb]
        else:
            self.p[pa] = pb
            self.size[pb] += self.size[pa]
        return True


class Solution:
    def numIslands2(self, m: int, n: int, positions: List[List[int]]) -> List[int]:
        uf = UnionFind(m * n)
        grid = [[0] * n for _ in range(m)]
        ans = []
        dirs = (-1, 0, 1, 0, -1)
        cnt = 0
        for i, j in positions:
            if grid[i][j]:
                ans.append(cnt)
                continue
            grid[i][j] = 1
            cnt += 1
            for a, b in pairwise(dirs):
                x, y = i + a, j + b
                if (
                    0 <= x < m
                    and 0 <= y < n
                    and grid[x][y]
                    and uf.union(i * n + j, x * n + y)
                ):
                    cnt -= 1
            ans.append(cnt)
        return ans