456789101112131415161718192021// Return #nodes that matches P or Q in the subtree.int countMatchesPQ(Node *root, Node *p, Node *q) { if (!root) return 0; int matches = countMatchesPQ(root->left, p, q) + countMatchesPQ(root->right, p, q); if (root == p || root == q) return 1 + matches; else return matches;} Node *LCA(Node *root, Node *p, Node *q) { if (!root || !p || !q) return NULL; if (root == p || root == q) return root; int totalMatches = countMatchesPQ(root->left, p, q); if (totalMatches == 1) return root; else if (totalMatches == 2) return LCA(root->left, p, q); else /* totalMatches == 0 */ return LCA(root->right, p, q);}
Bottom to up. Post-order traversal
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}
Lowest common ancestor of a binary tree Part I