Lowest Common Ancestor of a Binary Tree

Question

This is a popular interview question and the only article I can find on the topic is one from TopCoder. Unfortunately for me, it looks overly complicated from an interview answe

Answer 1

The bottom up approach described here is an O(n) time, O(1) space approach:

http://www.leetcode.com/2011/07/lowest-common-ancestor-of-a-binary-tree-part-i.html

Node *LCA(Node *root, Node *p, Node *q) {
  if (!root) return NULL;
  if (root == p || root == q) return root;
  Node *L = LCA(root->left, p, q);
  Node *R = LCA(root->right, p, q);
  if (L && R) return root;  // if p and q are on both sides
  return L ? L : R;  // either one of p,q is on one side OR p,q is not in L&R subtrees
}