0200：岛屿数量（★）

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class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        def find(x):
            if f[x] != x:
                f[x] = find(f[x])
            return f[x]

        def union(x, y):
            f[find(x)] = find(y)

        m, n = len(grid), len(grid[0])
        f = list(range(m*n))
        for i, j in product(range(m), range(n)):
            if grid[i][j] == '1':
                if i and grid[i-1][j] == '1':
                    union(i*n+j,(i-1)*n+j)
                if j and grid[i][j-1] == '1':
                    union(i*n+j,i*n+j-1)
        return sum(find(x)==x for x in range(m*n) if grid[x//n][x%n]=='1')