leetcode 669. Trim a Binary Search Tree（修剪二叉搜索树）

Given the root of a binary search tree and the lowest and highest boundaries as low and high, trim the tree so that all its elements lies in [low, high]. Trimming the tree should not change the relative structure of the elements that will remain in the tree (i.e., any node’s descendant should remain a descendant). It can be proven that there is a unique answer.

Return the root of the trimmed binary search tree. Note that the root may change depending on the given bounds.

Example 2:

Input: root = [3,0,4,null,2,null,null,1], low = 1, high = 3
Output: [3,2,null,1]

给出范围下限low，上限high，删掉low～high范围外的节点，同时要保持BST的结构

思路：
BST的特征是左子树节点值 < root值 < 右子树节点值
如果root.val < low的话，那么整个左子树和root都要剪掉，只需要返回修剪右子树的结果。同理root.val > high时，整个右子树和root都要剪掉，只要返回修剪左子树的结果。
如果root.val在范围内，那root不用动，继续修剪它的左子树和右子树。
节点是null时返回null。
递归实现

    public TreeNode trimBST(TreeNode root, int low, int high) {
        if(root == null) return null;
        
        if(root.val < low) return trimBST(root.right, low, high);
        if(root.val > high) return trimBST(root.left, low, high);
        
        root.left = trimBST(root.left, low, high);
        root.right = trimBST(root.right, low, high);
        
        return root;
     }