/**
 * 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) {
        HashSet<Integer> hashSet = new HashSet<Integer>();

        // You can also use a queue for breadth-first
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode curr = stack.pop();

            int difference = k - curr.val;

            if (hashSet.contains(difference)) {
                return true;
            }
            hashSet.add(curr.val);

            if (curr.left != null) {
                stack.push(curr.left);
            }
            if (curr.right != null) {
                stack.push(curr.right);
            }
        }

        return false;
    }
}