Problem

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

My Solution

class Solution {

    Map<Integer, Integer> map = new HashMap();
    boolean flag = false;

    public void preOrder(TreeNode node, int k, int order) {
        if (node != null) {
            if (map.containsValue(k - visit(node))) {
               flag = true;
               return;
            } else {
                map.put(order, visit(node));
            }
            preOrder(node.left, k, 2 * order);
            preOrder(node.right, k, 2 * order + 1);
        }
    }

    public int visit(TreeNode node) {
        return node.val;
    }

    public boolean findTarget(TreeNode root, int k) {
        preOrder(root, k, 1);
        return flag;
    }
}

Great Solution

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);
    }
}