0235. Lowest Common Ancestor of a Binary Search Tree.js

// 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).”
// Given binary search tree:  root = [6,2,8,0,4,7,9,null,null,3,5]
//
// 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.
//
// Note:
//
// All of the nodes' values will be unique.
// p and q are different and both values will exist in the BST.
/**
 * 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}
 */

// 1) Recursion
const lowestCommonAncestor1 = (root, p, 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;
};

// 2) Iteration
const lowestCommonAncestor = (root, p, q) => {
  while (root != null) {
    if (p.val > root.val && q.val > root.val) root = root.right;
    else if (p.val < root.val && q.val < root.val) root = root.left;
    else return root;
  }
};