We use a post-order traversal (bottom-up). If a node is p or q, we return it. If a node receives non-null values from both its left and right children, it must be the LCA.

1. Base case: If root is None, or root is p or q, return root.
2. Recursively search in the left subtree: left = lca(root.left, p, q).
3. Recursively search in the right subtree: right = lca(root.right, p, q).
4. If both left and right are not None:
   • This means p and q are in different subtrees, so root is the LCA.
5. If only one is not None, return that non-null value (the found p or q).
6. If both are None, return None.

• Time Complexity: O(N).
• Space Complexity: O(H).

def lowest_common_ancestor(root, p, q):
    if not root or root == p or root == q:
        return root
        
    left = lowest_common_ancestor(root.left, p, q)
    right = lowest_common_ancestor(root.right, p, q)
    
    if left and right:
        return root
        
    return left if left else right