988. Smallest String Starting From Leaf

-
https://leetcode.com/problems/smallest-string-starting-from-leaf/

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:

    1. The number of nodes in the given tree will be between 1 and 8500.
    2. Each node in the tree will have a value between 0 and 25.