Find Bottom Left Tree Value LeetCode solution in Python

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# Definition for a binary tree node.
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# class TreeNode:
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#     def __init__(self, val=0, left=None, right=None):
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#         self.val = val
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#         self.left = left
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#         self.right = right
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class Solution:
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    def findBottomLeftValue(self, root: Optional[TreeNode]) -> int:
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        res = root.val
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        stack = [(0, root)]
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        prev_d = 0
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        while stack:
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            curr_d, curr_v = stack.pop(0)
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            if curr_v.left:
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                stack.append((curr_d + 1, curr_v.left))
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                if prev_d != curr_d + 1:
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                    res = curr_v.left.val
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                    prev_d = curr_d + 1
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            if curr_v.right:
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                stack.append((curr_d + 1, curr_v.right))
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                if prev_d != curr_d + 1:
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                    res = curr_v.right.val
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                    prev_d = curr_d + 1
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        return res
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