bool checkPossibility(vector<int>& nums) {

        int cnt = ;                                                                    //如果存在两个以上的异常值则直接返回false

        for(int i = ; i < nums.size() && cnt<= ; i++){

            if(nums[i-] > nums[i]){                                                    //存在异常值

                cnt++;

                if(i-< || nums[i-] <= nums[i])nums[i-] = nums[i];                    //i-2处理第一个边界值，这种||技巧经常用到

                else nums[i] = nums[i-];                                                //have to modify nums[i]

            }

        }

        return cnt<=;

    } 

The code works as recursion.

If the root value in the range [L, R]

      we need return the root, but trim its left and right subtree;

else if the root value < L

      because of binary search tree property, the root and the left subtree are not in range;

      we need return trimmed right subtree.

else

      similarly we need return trimmed left subtree.

Without freeing memory

class Solution {

public:

    TreeNode* trimBST(TreeNode* root, int L, int R) {

        if (root == NULL) return NULL;

        if (root->val < L) return trimBST(root->right, L, R);

        if (root->val > R) return trimBST(root->left, L, R);

        root->left = trimBST(root->left, L, R);

        root->right = trimBST(root->right, L, R);

        return root;

    }

};