LeetCode - Two Sum IV - Input is a BST

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

只要是两数之和的题，一定要记得用哈希表来做，这道题只不过是把数组变成了一棵二叉树而已

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