https://leetcode.com/problems/insert-into-a-binary-search-tree (recursive)

insert-into-a-binary-search-tree.cpp

class Solution {
public:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        if (root == nullptr) return new TreeNode(val);
        if (val < root->val) {
            root->left = insertIntoBST(root->left, val);
        } else {
            root->right = insertIntoBST(root->right, val);
        }
        return root;
    }
};