public class Solution
extends Object
988 - Smallest String Starting From Leaf\.
Medium
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"
**Example 2:**
**Input:** root = [25,1,3,1,3,0,2]
**Output:** "adz"
**Example 3:**
**Input:** root = [2,2,1,null,1,0,null,0]
**Output:** "abc"
**Constraints:**
* The number of nodes in the tree is in the range `[1, 8500]`.
* `0 <= Node.val <= 25`