# 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
```

## Analysis

<https://leetcode.com/articles/two-sum-iv/>

**BFS + HashSet**

广度优先搜索，level order traversal：每一个节点遍历时，在hashset里面找是否存在target - currentNode.val，每一层的左右子节点依次放入queue，同时在hashset里面存入当前遍历的节点：

> 1. Remove an element,pp, from the front of thequeuequeue.
> 2. Check if the elementk-pk−palready exists in thesetset. If so, return True.
> 3. Otherwise, add this element,ppto thesetset. Further, add the right and the left child nodes of the current node to the back of thequeuequeue.
> 4. Continue steps 1. to 3. till thequeuequeuebecomes empty.
> 5. Return false if thequeuequeuebecomes empty.

**BST In-order Traversal + Two Pointers**

先in-order遍历BST，输出到一个list，再对这个list用2 sum的方法找target即可，因为list排序过，因此用two pointers很方便。

## Solution

BFS + HashSet - Time: O(n), Space O(n)

```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) {
        Queue<TreeNode> queue = new LinkedList();
        Set<Integer> set = new HashSet();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
            if (set.contains(k - node.val)) {
                return true;
            }
            set.add(node.val);
        }
        return false;
    }
}
```

BST inorder + Two Pointers

```java
public class Solution {
    public boolean findTarget(TreeNode root, int k) {
        List < Integer > list = new ArrayList();
        inorder(root, list);
        int l = 0, r = list.size() - 1;
        while (l < r) {
            int sum = list.get(l) + list.get(r);
            if (sum == k)
                return true;
            if (sum < k)
                l++;
            else
                r--;
        }
        return false;
    }
    public void inorder(TreeNode root, List < Integer > list) {
        if (root == null)
            return;
        inorder(root.left, list);
        list.add(root.val);
        inorder(root.right, list);
    }
}
```

Recursive + HashSet

```java
public class Solution {
    public boolean findTarget(TreeNode root, int k) {
        Set < Integer > set = new HashSet();
        return find(root, k, set);
    }
    public boolean find(TreeNode root, int k, Set < Integer > set) {
        if (root == null)
            return false;
        if (set.contains(k - root.val))
            return true;
        set.add(root.val);
        return find(root.left, k, set) || find(root.right, k, set);
    }
}
```