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### by Sumit Chourasia | Oct 08, 2020 | Category :coding | Tags : algorithmbinary-treedata-structureeasyleetcodetree #### Two Sum IV - Input is a BST - Tree - Easy - LeetCode

Given the root of a Binary Search Tree and a target number k, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input: root = [5,3,6,2,4,null,7], k = 9
Output: true
Example 2:

Input: root = [5,3,6,2,4,null,7], k = 28
Output: false
Example 3:

Input: root = [2,1,3], k = 4
Output: true
Example 4:

Input: root = [2,1,3], k = 1
Output: false
Example 5:

Input: root = [2,1,3], k = 3
Output: true

Constraints:

The number of nodes in the tree is in the range [1, 104].
-104 <= Node.val <= 104
root is guaranteed to be a valid binary search tree.
-105 <= k <= 105

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     public int val;
*     public TreeNode left;
*     public TreeNode right;
*     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
*         this.val = val;
*         this.left = left;
*         this.right = right;
*     }
* }
*/
public class Solution {
public bool FindTarget(TreeNode root, int k) {
var set = new HashSet<int>();
return Helper(root,set,k);
}

private bool Helper(TreeNode root, HashSet<int> set, int k){
if(root == null){
return false;
}

if(set.Contains(k-root.val)){
return true;
}