653 Two Sum IV - Input is a BST: 05/17

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

• [x] ### use set to store the difference between the target value and the current value, then when you encounter the difference return true, else return false

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    Set<Integer> set = new HashSet<>();
    public boolean findTarget(TreeNode root, int k) {
        if(root == null) {
            return false;
        }
        if (set.contains(root.val)) {
            return true;
        }else{
            set.add(k - root.val);
        }
        return findTarget(root.left, k) || findTarget(root.right, k);
    }
}