Number of Islands LeetCode solution in Python

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# Runtime: 353 ms (Top 84.15%) | Memory: 28.7 MB (Top 5.01%)
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seen = set()
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def dfs(i, j, m, n, grid):
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    global seen
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    if i < 0 or i >= m or j < 0 or j >= n:
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        return
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    if (i, j) in seen:
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        return
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    seen.add((i, j))
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    if grid[i][j] == "1":
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        dfs(i, j + 1, m, n, grid)
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        dfs(i, j - 1, m, n, grid)
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        dfs(i + 1, j, m, n, grid)
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        dfs(i - 1, j, m, n, grid)
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class Solution:
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    def numIslands(self, grid: List[List[str]]) -> int:
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        global seen
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        m = len(grid)  # no of rows
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        n = len(grid[0])  # no of columns
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        count = 0
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        for i in range(m):
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            for j in range(n):
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                if (i, j) not in seen and grid[i][j] == "1":
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                    dfs(i, j, m, n, grid)
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                    count += 1
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        seen.clear()
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        return count