leetcode-653-easy

Two Sum IV - Input is a BST

Given the root of a binary search tree and an integer k, return true if there exist two elements in the BST such that their sum is equal to k, or false otherwise.

Example 1:

Input: root = [5,3,6,2,4,null,7], k = 9
Output: true
Example 2:

Input: root = [5,3,6,2,4,null,7], k = 28
Output: false
Constraints:

The number of nodes in the tree is in the range [1, 104].
-104 <= Node.val <= 104
root is guaranteed to be a valid binary search tree.
-105 <= k <= 105

思路一：遍历二叉树，值用 set 存储，判断是否存在符合条件的解

    public boolean findTarget(TreeNode root, int k) {
        Set<Integer> set = new HashSet<>();
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            if (node.left != null) {
                queue.offer(node.left);
            }

            if (node.right != null) {
                queue.offer(node.right);
            }

            if (set.contains(k - node.val)) {
                return true;
            } else {
                set.add(node.val);
            }
        }

        return false;
    }