Cut Off Trees for Golf Event LeetCode solution in Python

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from collections import deque
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class Solution:
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    def cutOffTree(self, forest: List[List[int]]) -> int:
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        a = []
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        n = len(forest)
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        m = len(forest[0])
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        for i in range(n):
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            for j in range(m):
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                if forest[i][j] > 1:
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                    a.append(forest[i][j])
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        a.sort()
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        s = 0
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        ux = 0
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        uy = 0
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        for h in a:
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            dist = [[None] * m for i in range(n)]
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            q = deque()
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            q.append((ux, uy))
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            dist[ux][uy] = 0
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            while q:
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                ux, uy = q.popleft()
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                if forest[ux][uy] == h:
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                    break
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                d = [(-1, 0), (0, 1), (1, 0), (0, -1)]
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                for dx, dy in d:
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                    vx = ux + dx
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                    vy = uy + dy
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                    if vx < 0 or vx >= n or vy < 0 or vy >= m:
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                        continue
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                    if forest[vx][vy] == 0:
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                        continue
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                    if dist[vx][vy] is None:
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                        q.append((vx, vy))
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                        dist[vx][vy] = dist[ux][uy] + 1
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            if forest[ux][uy] == h:
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                s += dist[ux][uy]
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            else:
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                return -1
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        return s