feat: Lowest Common Ancestor of a Binary Search Tree

Author: hongbo-miao

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

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+// 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 */
+function lowestCommonAncestor(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 */
+function 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;
+  }
+}