Get Keys In Binary Search Tree In Given Range - Easy

/**
 * public class TreeNode {
 *   public int key;
 *   public TreeNode left;
 *   public TreeNode right;
 *   public TreeNode(int key) {
 *     this.key = key;
 *   }
 * }
 */
public class Solution {
  public List<Integer> getRange(TreeNode root,int min,int max) {
    // Write your solution here
    List<Integer> res = new ArrayList<>();
    if(root == null || min > max) {
      return res;
    }
    helper(root,min,max,res);
    return res;
  }
  
  public void helper(TreeNode root,int max,List<Integer> res) {
    if(root == null) {
      return;
    }
    if(root.key > min) {
      helper(root.left,res);
    }
    if(root.key >= min && root.key <= max) {
      res.add(root.key);
    }
    if(root.key < max){
      helper(root.right,res);
    }
  }
}