Smallest String Starting From Leaf

Given a binary tree where each node has a letter (a-z), find the lexicographically smallest string that starts from a leaf and ends at the root.

Examples
Input: [0,1,2,3,4,3,4]
Output: "dba"
Hints

Smallest String Starting From Leaf

Given a binary tree where each node has a letter (a-z), find the lexicographically smallest string that starts from a leaf and ends at the root.