988. 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