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`

.