Smallest String Starting From Leaf In Binary Tree

Smallest String Starting From Leaf

Difficulty: Medium
Topics: String, Backtracking, Tree, Depth-First Search, Binary Tree
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.
Example 1:


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

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