235 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.

Solution

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if ( root == NULL ) return NULL;
        if ( root == p or root == q ) return root;
        if ( root->val < p->val and root->val < q->val ) return lowestCommonAncestor(root->right, p, q);
        if ( root->val > p->val and root->val > q->val ) return lowestCommonAncestor(root->left, p, q);
        return root;
    }
};