653 Two Sum IV - Input is a BST

Given the root of a binary search tree and an integer k, return true if there exist two elements in the BST such that their sum is equal to k, or false otherwise.

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

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean findTarget(TreeNode root, int k) {
        Set<Integer> set = new HashSet<Integer>();
        return helper(root, k, set);
    }

    private boolean helper(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 helper(root.left, k , set) || helper(root.right, k, set);
    }
}

// TC:O(n)

// SC:O(n)