653 Two Sum
Given a binary search tree and a target number, rerturn true if there exist two elements in the BST such that their sum is equal to the given target.

Approach 1 HashSet

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);
    }
}

Approach 2 BFS and HashSet

public class Solution {
    public boolean findTarget(TreeNode root, int k) {
        Set < Integer > set = new HashSet();
        Queue < TreeNode > queue = new LinkedList();
        queue.add(root);
        while (!queue.isEmpty()) {
            if (queue.peek() != null) {
                TreeNode node = queue.remove();
                if (set.contains(k - node.val))
                    return true;
                set.add(node.val);
                queue.add(node.right);
                queue.add(node.left);
            } else
                queue.remove();
        }
        return false;
    }
}