A leaf is a node that has no left child and no right child. Count how many leaves the given binary tree has.
Input format
One line: the binary tree as a level-order array.
The tree is encoded on ONE line as a space-separated level-order (breadth-first) array. The token null marks a missing child; the children of a null are omitted from the array. An empty tree is written as the single token null.
Output format
A single integer: the number of leaf nodes (0 for an empty tree).
Constraints
- The tree has between 0 and 1000 nodes.
- Each node value is an integer with absolute value at most 1000.