leetcode-并查集

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        f = {}
        def find(x):
            f.setdefault(x,x)
            if f[x]!=x:
                f[x] = find(f[x])
            return f[x]
        def union(x,y):
            f[find(y)] = find(x)
        if not board or not board[0]:
            return
        row,col = len(board),len(board[0])
        dummy = row*col
        for i in range(row):
            for j in range(col):
                if board[i][j] == "O":
                    if i == 0 or i == row - 1 or j == 0 or j == col - 1:
                        union(i * col + j, dummy)
                    else:
                        for x, y in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
                            if board[i + x][j + y] == "O":
                                union(i * col + j, (i + x) * col + (j + y))
                                
        for i in range(row):
            for j in range(col):
                if find(dummy) == find(i * col + j):
                    board[i][j] = "O"
                else:
                    board[i][j] = "X"

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        f = {}
        def find(x):
            f.setdefault(x,x)
            if f[x]!=x:
                f[x] = find(f[x])
            return f[x]
        def union(x,y):
            f[find(y)] = find(x)
            
        if not grid:
            return 0
        row,col =len(grid),len(grid[0])
        for i in range(row):
            for j in range(col):
                if grid[i][j] == "1":
                    for x, y in [[-1, 0], [0, -1]]:
                        tmp_i = i + x
                        tmp_j = j + y
                        if 0 <= tmp_i < row and 0 <= tmp_j < col and grid[tmp_i][tmp_j] == "1":
                            union(tmp_i * col + tmp_j, i * col + j)
        res = set()
        for i in range(row):
            for j in range(col):
                if grid[i][j] == "1":
                    res.add(find(col*i+j))
        return len(res)