human
Solve in Python:
Given the root of a binary tree, each node has a value from 0 to 25 representing the letters 'a' to 'z': a value of 0 represents 'a', a value of 1 represents 'b', and so on.
Find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.
(As a reminder, any shorter prefix of a string is lexicographically smaller: for example, "ab" is lexicographically smaller than "aba".  A leaf of a node is a node that has no children.)

 



Example 1:

Input: [0,1,2,3,4,3,4]
Output: "dba"

Example 2:

Input: [25,1,3,1,3,0,2]
Output: "adz"

Example 3:

Input: [2,2,1,null,1,0,null,0]
Output: "abc"

 
Note:

The number of nodes in the given tree will be between 1 and 8500.
Each node in the tree will have a value between 0 and 25.
assistant
#Here is Python program to find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
import string
class Solution:
    def smallestFromLeaf(self, root: TreeNode) -> str:   
        def pathToLeaf(root, letters):
            nonlocal smallest
            if root:
                letters+=chr(root.val + ord('a'))
                if root.right == None and root.left == None:
                    reversed_letters = letters[::-1]
                    if smallest > reversed_letters:
                        smallest =  reversed_letters
                else:
                    pathToLeaf(root.left, letters)
                    pathToLeaf(root.right,  letters)
        
        smallest = \"~\"
        pathToLeaf(root, \"\")
        return smallest