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class Solution:
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    res = "z" * 13  # init max result, tree depth,  12< log2(8000) < 13
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    def smallestFromLeaf(self, root: TreeNode) -> str:
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        def helper(node: TreeNode, prev):
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            prev = chr(97 + node.val) + prev
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            if not node.left and not node.right:
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                self.res = min(self.res, prev)
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                return
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            if node.left:
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                helper(node.left, prev)
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            if node.right:
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                helper(node.right, prev)
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        helper(root, "")
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        return self.res