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.)