235. Lowest Common Ancestor of a Binary Search Tree

Problem

Tags: Tree, Depth-First Search, Binary Search Tree, Binary 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 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, 10^5].
• -10^9 <= Node.val <= 10^9
• All Node.val are unique.
• p != q
• p and q will exist in the BST.

Code

JS

// 235. Lowest Common Ancestor of a Binary Search Tree (6/12/53748)
// Runtime: 136 ms (15.89%) Memory: 48.54 MB (93.21%) 

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */

/**
 * @param {TreeNode} root
 * @param {TreeNode} p
 * @param {TreeNode} q
 * @return {TreeNode}
 */
function lowestCommonAncestor(root, p, q) {
    if (root.val > p.val && root.val > q.val) return lowestCommonAncestor(root.left, p, q);
    else if (root.val < p.val && root.val < q.val) return lowestCommonAncestor(root.right, p, q);
    else return root;
}