# 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:**

![](https://assets.leetcode.com/uploads/2020/09/21/sum_tree_1.jpg)

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

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/09/21/sum_tree_2.jpg)

```
**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

```java
/**
 * 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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