LeetCode 701. Insert into a Binary Search Tree (javascript)

You are given the root node of a binary search tree (BST) and a value to insert into the tree. Return the root node of the BST after the insertion. It is guaranteed that the new value does not exist in the original BST.

Notice that there may exist multiple valid ways for the insertion, as long as the tree remains a BST after insertion. You can return any of them.

Example 1:

Input: root = [4,2,7,1,3], val = 5
Output: [4,2,7,1,3,5]
Explanation: Another accepted tree is:

Example 2:

Input: root = [40,20,60,10,30,50,70], val = 25
Output: [40,20,60,10,30,50,70,null,null,25]

Example 3:

Input: root = [4,2,7,1,3,null,null,null,null,null,null], val = 5
Output: [4,2,7,1,3,5]

Constraints:

The number of nodes in the tree will be in the range [0, 10⁴].
-10⁸ <= Node.val <= 10⁸
All the values Node.val are unique.
-10⁸ <= val <= 10⁸
It’s guaranteed that val does not exist in the original BST.

Solution: (Recursion)

Time complexity: O(logn ~ n)
Space complexity: O(logn ~ n)

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} val
 * @return {TreeNode}
 */
var insertIntoBST = function(root, val) {
    // empty tree case:
    if (root === null) return new TreeNode(val);
    if (val > root.val) {
        root.right = insertIntoBST(root.right, val);
    } else {
        root.left = insertIntoBST(root.left, val);
    }
    return root;
};