669. Trim a Binary Search Tree

/**
 * 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 the value of this node is less than L, return the root of the trimmed right sub tree
        if (root.val < L) {
            return trimBST(root.right, L, R);
        } 
        
        // If the value of this node is greater than R, return the root of the trimmed left sub tree
        if (root.val > R) {
            return trimBST(root.left, L, R);
        }
        
        // If the value of this node does not need to be deleted, we need to pass this recursive call downwards to both sides
        root.left = trimBST(root.left, L, R);
        root.right = trimBST(root.right, L, R);
        
        // All the processing of the subtrees done, now return this node
        return root;
    }
}