0653. Two Sum IV - Input is a BST
https://leetcode.com/problems/two-sum-iv-input-is-a-bst
Description
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
ac
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean findTarget(TreeNode root, int k) {
return helper(root, new HashSet<Integer>(), k);
}
private boolean helper(TreeNode root, Set<Integer> set, int k) {
// exit
if (root == null) return false;
if (set.contains(k - root.val)) return true;
set.add(root.val);
return helper(root.left, set, k) || helper(root.right, set, k);
}
}
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