Lowest Common Ancestor (BT / BST / N-ary)

LeetCode 235 (BST), 236 (binary tree), and 1644 (with optional nodes). The N-ary variant is Meta's preferred follow-up. Requirements BST LCA: walk down comparing values. Binary-tree LCA: post-order — if both children return non-null, current is LCA. Optional-node variant: must confirm both targets exist before returning. N-ary variant: same post-order pattern; collect non-null hits across all children. Follow-up: nodes have parent pointers — two-pointer chase to common ancestor. Notes Confirm up front whether either target may be absent — changes return contract. Some interviewers ask for both recursive and parent-pointer iterative solutions. Preparation Write LCA-binary-tree, LCA-BST, LCA-with-parent-pointer from memory. Practice the N-ary post-order variant — Meta asks for it explicitly.