Invert Binary Tree

Mirror the tree: swap the left and right child of every node. Print the inverted tree.

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 inverted tree in the same level-order format.

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

Think about it: swap the children of the root, then invert both subtrees — recursion or a queue both work. You also need to print a tree in level order: BFS, printing null for missing children, then drop the trailing nulls.