You are given the root of a binary tree. Your task is to count and return the number of leaf nodes in the binary tree. A leaf node is a node with no left or right child.
Input: root = [1,2,3,4,5]
Output: 3
Explanation: The leaf nodes are 4, 5, and 3.
Input: root = [1]
Output: 1
Explanation: The root node is also a leaf node.
Input: root = []
Output: 0
Explanation: The tree contains no nodes.
Sign in to write, run, and submit your solution against the full test suite.