669. 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

Analysis:
If current root's val < L, we skip it and try to search its right child. If current root.val > R, we skip it and try to search its left child.
Solution:

class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) return null;
        TreeNode left = trimBST(root.left, L, R);
        TreeNode right = trimBST(root.right, L, R);
        if (root.val < L) return right; 
        if (root.val > R) return left;
        root.left = left;
        root.right = right;
        return root;
    }
}