[LeetCode] Two Sum IV - Input is a BST

转载于 2018-05-13 16:06:17 发布 · 59 阅读

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原文链接：https://segmentfault.com/a/1190000014836875

本文介绍了一种在二叉搜索树中查找是否存在两个元素之和等于给定目标值的方法。通过广度优先搜索结合哈希集合，高效地解决了这一问题。

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Problem

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

Example 1:
Input:

Target = 9

Output: True
Example 2:
Input:

Target = 28

Output: False

Solution

/**
 * 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) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        Set<Integer> set = new HashSet<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            if (set.contains(k-cur.val)) return true;
            set.add(cur.val);
            if (cur.left != null) queue.offer(cur.left);
            if (cur.right != null) queue.offer(cur.right);
        }
        return false;
    }
}