Binary Tree Inorder Traversal

Print the values of a binary tree in inorder: left subtree, then the node, then right subtree.

Tree format. A binary tree is given on one line in level order (top to bottom, left to right), with null for a missing child — the same way most interview platforms show trees. Trailing nulls are left out, and an empty tree is the single word null. Example: 3 9 20 null null 15 7 is

    3
   / \
  9   20
     /  \
    15   7

Input: one tree.

Output: the inorder sequence on one line, or empty for an empty tree.

Constraints: 0 ≤ nodes ≤ 10⁴, −1000 ≤ value ≤ 1000. The tree has at most 10⁴ nodes and its height is at most 500, so recursion is safe in every language.

Practise both ways: the recursive version is three lines. Then write the iterative version with an explicit stack — interviewers love asking for it.