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def is_leaf(x):
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    return x.left is None and x.right is None
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4

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class Solution:
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    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
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        if root is None:
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            return 0
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        if root.left and is_leaf(root.left):
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            left = root.left.val
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        else:
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            left = self.sumOfLeftLeaves(root.left)
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        return left + self.sumOfLeftLeaves(root.right)