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);
    }
}