經典題目：給定一個二叉搜索樹，插入一個值為val的新節點

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* insertIntoBST(TreeNode* root, int val) {
        
 
if(root==NULL){
            root=new TreeNode(val);
            return root;
        }
        if(root->val<val){
            root->right=insertIntoBST(root->right,val);
        }else{
            root->left=insertIntoBST(root->left,val);
        }
        return root;
    }
};

701. Insert into a Binary Search Tree