message: Update closest-leaf-in-a-binary-tree.py
diff: # Time: O(n)
  # Space: O(n)
  +# Given a binary tree where every node has a unique value, and a target key k,
  +# find the value of the closest leaf node to target k in the tree.
  +#
  +# Here, closest 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 closest leaf node to the target of 1.
  +# Example 2:
  +#
  +# Input:
  +# root = [1], k = 1
  +# Output: 1
  +#
  +# Explanation: The closest 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 closest to the node with value 2.
  +# Note:
  +# - root represents a binary tree with at least 1 node and at most 1000 nodes.
  +# - Every node has a unique node.val in range [1, 1000].
  +# - There exists some node in the given binary tree for which node.val == k.
  +
  # Definition for a binary tree node.
  # class TreeNode(object):
  # def __init__(self, x):
  