You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters 'a' to 'z'.
Return 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.
Input: root = [0,1,2,3,4,3,4] Output: "dba"
Input: root = [25,1,3,1,3,0,2] Output: "adz"
Input: root = [2,2,1,null,1,0,null,0] Output: "abc"
- The number of nodes in the tree is in the range
[1, 8500]. 0 <= Node.val <= 25
# 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 = rightclassSolution: defsmallestFromLeaf(self, root: Optional[TreeNode]) ->str: returnself.dfs("", root) defdfs(self, s: str, root: Optional[TreeNode]) ->str: ifrootisNone: returnss=chr(root.val+97) +sifroot.leftisNone: returnself.dfs(s, root.right) elifroot.rightisNone: returnself.dfs(s, root.left) else: returnmin(self.dfs(s, root.left), self.dfs(s, root.right))