LeetCode--Trim a Binary Search Tree

Ideas:

    Traverse the entire tree in preorder, and jump directly to the left or right child of the node that is not in the threshold.

/**
 * 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<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;
    }   
}