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 has no children.   **Example 1:** **Input:** root = [0,1,2,3,4,3,4] **Output:** "dba" ``` **Example 2:** **Input:** root = [25,1,3,1,3,0,2] **Output:** "adz" ``` **Example 3:** **Input:** root = [2,2,1,null,1,0,null,0] **Output:** "abc" ```   **Constraints:** - The number of nodes in the tree is in the range `[1, 8500]`. - `0 <= Node.val <= 25`