Trim a Binary Search Tree

Problem

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

Solution

Recursive Solution

public class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) return root;
        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;
    }
}

Analysis

This is a typical tree traversal problem with typical recursive solution
We know that we have to change root node if that root has vale outside of L and R
If root.val > R, we need to set the root as root.left cause root.right.val > root.val
Same idea when root.val < L
If root.val is in the given range, we set root.left and root.right recursively calling the method