Count Leaf Nodes

Given the root of a binary tree, return the **number of leaf nodes**.

A **leaf node** is a node with no children (both left and right are null).

Input: root = [1,2,3,4,5]

Output: 3

Leaves are 4, 5, and 3.

Input: root = [1]

Output: 1

Root itself is the only leaf.

Time

O(n)

every node is visited.

Space

O(h)

recursive stack.

#1 Five-node tree

Input: [1,2,3,4,5]

Expected: 3

#2 Single node

Input: [1]

Expected: 1

#3 Root + two leaves

Input: [1,2,3]

Expected: 2

#4 Left-skewed branch

Input: [1,2,null,3]

Expected: 1