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