669. Trim a Binary Search Tree

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:

L
R
L
R

    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;        
    }