# 653. Two Sum IV - Input is a BST

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

**Example 1:**

```
Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 9

Output: True
```

**Example 2:**

```
Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 28

Output: False
```

{% hint style="info" %}
用一個hashset紀錄已經traverse過的數字，在recursive helper function裡如果在hashset裡找得到目標值減去當前節點值的數，就可以直接傳回true。
{% endhint %}

```cpp
bool helper(TreeNode* root, unordered_set<int>& st, int k) {
    if (!root) return false;
    if (st.count(k - root->val)) return true;
    st.insert(root->val);
    return helper(root->left, st, k) || helper(root->right, st, k);
}
bool findTarget(TreeNode* root, int k) { // time: O(n); space: O(n)
    unordered_set<int> st;
    return helper(root, st, k);
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/tree/653.-two-sum-iv-input-is-a-bst.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.