Minimum Depth of Binary Tree LeetCode solution in Python

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class Solution(object):
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    def minDepth(self, root):
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        # Base case...
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        # If the subtree is empty i.e. root is NULL, return depth as 0...
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        if root is None:
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            return 0
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        # Initialize the depth of two subtrees...
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        leftDepth = self.minDepth(root.left)
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        rightDepth = self.minDepth(root.right)
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        # If the both subtrees are empty...
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        if root.left is None and root.right is None:
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            return 1
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        # If the left subtree is empty, return the depth of right subtree after adding 1 to it...
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        if root.left is None:
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            return 1 + rightDepth
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        # If the right subtree is empty, return the depth of left subtree after adding 1 to it...
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        if root.right is None:
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            return 1 + leftDepth
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        # When the two child function return its depth...
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        # Pick the minimum out of these two subtrees and return this value after adding 1 to it...
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        return min(leftDepth, rightDepth) + 1
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        # Adding 1 is the current node which is the parent of the two subtrees...