669. Trim a Binary Search Tree

简单题目，BST的搜索遍历

/**
 * 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==NULL)
            return root;
        while(root!=NULL&&((root->val)<L||(root->val)>R)){
            if((root->val)<L)
                root = root-> right;
            else if((root->val)>R)
                root = root-> left;
            else
            {}
        }


        if (root==NULL)
              return NULL;
        else{
            root->left = trimBST(root->left, L,R);
            root->right = trimBST(root->right, L, R);

            return root;
        }
    }
};