742. Closest Leaf in a Binary Tree

Given a binary tree where every node has a unique value , and a target key k , find the value of the nearest leaf node to target k in the tree.

Here, nearest to a leaf means the least number of edges travelled on the binary tree to reach any leaf of the tree. Also, a node is called a leaf if it has no children.

In the following examples, the input tree is represented in flattened form row by row. The actual root tree given will be a TreeNode object.

Example 1:

Input:

root = [1, 3, 2], k = 1
Diagram of binary tree:
          1
         / \
        3   2
Output:

 2 (or 3)
Explanation:

 Either 2 or 3 is the nearest leaf node to the target of 1.

Example 2:

Input:

root = [1], k = 1
Output:

 1
Explanation:

 The nearest leaf node is the root node itself.

Example 3:

Input:

root = [1,2,3,4,null,null,null,5,null,6], k = 2
Diagram of binary tree:
             1
            / \
           2   3
          /
         4
        /
       5
      /
     6
Output:

 3
Explanation:

 The leaf node with value 3 (and not the leaf node with value 6) is nearest to the node with value 2.

Note:

1. root represents a binary tree with at least 1 node and at most 1000 nodes.
2. Every node has a unique node.val in range [1, 1000] .
3. There exists some node in the given binary tree for which node.val == k .