Micro-Drill #205 — Invert binary tree

Tree Traversal Target: 5s

Swap left and right children recursively. The classic 'Homebrew' interview question. Must be instant.

def invertTree(root):
    if not root: return None
    root.left, root.right = invertTree(root.right), invertTree(root.left)
    return root

Type it from memory. Go.