In this post, we are going to solve the 235. Lowest Common Ancestor of a Binary Search Tree problem of Leetcode. This problem 235. Lowest Common Ancestor of a Binary Search Tree is a Leetcode easy level problem. Let’s see code, 235. Lowest Common Ancestor of a Binary Search Tree – Leetcode Solution.

Problem

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

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

Example 1 :

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes 2 and 8 is 6.

Example 2 :

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

Example 3 :

Input: root = [2,1], p = 2, q = 1
Output: 2

Constraints

• The number of nodes in the tree is in the range [2, 105].
• -109 <= Node.val <= 109
• All Node.val are unique.
• p != q
• p and q will exist in the BST.

Now, let’s see the code of 235. Lowest Common Ancestor of a Binary Search Tree – Leetcode Solution.

Lowest Common Ancestor of a Binary Search Tree – Leetcode Solution

235. Lowest Common Ancestor of a Binary Search Tree – Solution in Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(p.val < root.val && q.val < root.val){
            return lowestCommonAncestor(root.left,p,q);
        }else if(p.val > root.val && q.val > root.val){
            return lowestCommonAncestor(root.right,p,q);
        }else{
            return root;
        }
    }
}

235. Lowest Common Ancestor of a Binary Search Tree – Solution in C++

/**
 * 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 (p->val < root->val and q->val < root->val){
        return lowestCommonAncestor(root->left, p, q);
    }
    else if (p->val > root->val and q->val > root->val){
        return lowestCommonAncestor(root->right, p, q);
    }
    return root;
}
};

235. Lowest Common Ancestor of a Binary Search Tree – Solution in Python

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
        if root:
            if root.val > p.val and root.val > q.val:
                return self.lowestCommonAncestor(root.left, p, q)
            elif root.val < p.val and root.val < q.val:
                return self.lowestCommonAncestor(root.right, p, q)
            else:
                return root
        

Note: This problem 235. Lowest Common Ancestor of a Binary Search Tree is generated by Leetcode but the solution is provided by CodingBroz. This tutorial is only for Educational and Learning purpose.