Lowest Common Ancestor of a Binary Search Tree

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______6______

       /              \

    ___2__          ___8__

   /      \        /      \

   0      _4       7       9

         /  \

         3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

 1 /**

 2  * Definition for a binary tree node.

 3  * struct TreeNode {

 4  *     int val;

 5  *     struct TreeNode *left;

 6  *     struct TreeNode *right;

 7  * };

 8  */

 9 struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {

10     if(!root && !p && !q)

11       return 0;

12     if(root->val == p->val)

13       return p;

14     if(root->val == q->val)

15       return q;

16     if((root->val > p->val && root->val < q->val)||(root->val < p->val && root->val > q->val))

17       return root;

18     if(root->val > p->val && root->val > q->val)

19       return lowestCommonAncestor(root->left,p,q);

20     if(root->val < p->val && root->val < q->val)

21       return lowestCommonAncestor(root->right,p,q);

22     return 0;

23 }