669. Trim a Binary Search Tree - Easy

Given a binary search tree and the lowest and highest boundaries as?L?and?R,trim the tree so that all its elements lies in?[L,R]?(R >= L). You might need to change the root of the tree,so the result should return the new root of the trimmed binary search tree.

Example 1:

Input: 
    1
   /   0   2

  L = 1
  R = 2

Output: 
    1
             2

?

Example 2:

Input: 
    3
   /   0   4
       2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

?

M1: recursion

如果root.val比R大，说明要trim掉右边的节点，要返回的subtree在root的左边，即递归到root.left；

如果root.val比L小，说明要trim掉左边的节点，要返回的subtree在root的右边，递归到root.right；

如果root.val在[L,R]，说明有可能左右两边都需要trim，分别递归左右节点

time: O(n),space: O(height)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode trimBST(TreeNode root,int L,int R) {
        if(root == null) {
            return root;
        }
        if(root.val > R) {
            return trimBST(root.left,L,R);
        }
        if(root.val < L) {
            return trimBST(root.right,R);
        }
        root.left = trimBST(root.left,R);
        root.right = trimBST(root.right,R);
        
        return root;
    }
}