0988. Smallest String Starting From Leaf
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https://leetcode.com/problems/smallest-string-starting-from-leaf
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
