输入: 
    1
   / \
  0   2
​
  L = 1
  R = 2
​
输出: 
    1
      \
       2
示例 2:
​
输入: 
    3
   / \
  0   4
   \
    2
   /
  1
​
  L = 1
  R = 3
​
输出: 
      3
     / 
   2   
  /
 1

/**
 * 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* trimBST(TreeNode* root, int L, int R) {
        if(!root ) return NULL;
        if(root->val < L) 
            return root->right? trimBST(root->right, L, R) :NULL;
        if(root->val > R)
            return root->left? trimBST(root->left, L, R) :NULL;       
        if(root->val==L)
            root->left=NULL;
        if(root->val==R)
            root->right=NULL;
​
       // TreeNode* ptr=root;
        if(root->left) root->left=trimBST(root->left,L, R);
        if(root->right) root->right=trimBST(root->right, L, R);
        return root;
​
​
    }
};