Binary Search Tree Iterator

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Threading.Tasks;
 6 
 7 namespace LeetCode
 8 {
 9     class BSTIterator
10     {
11         Stack<TreeNode> stack = new Stack<TreeNode>();
12         public BSTIterator (TreeNode root)
13         {
14             while (root!=null)
15             {
16                 stack.Push(root);
17                 root = root.left;
18             }
19         }
20 
21         public bool HasNext()
22         {
23             return stack.Count > 0;
24         }
25 
26         public int Next()
27         {
28             TreeNode node = stack.Pop();
29             int res = node.val;
30             if (node.right != null)
31             {
32                 node = node.right;
33                 while (node != null)
34                 {
35                     stack.Push(node);
36                     node = node.left;
37                 }
38             }
39             return res;
40         }
41     }
42 }