leetcode-653-Two-Sum-IV-Input-is-a-BST

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描述

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:

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Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 9

Output: True

Example 2:

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Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 28

Output: False

分析

可以使用哈希集合存树节点的值，遍历二叉树可以使用深度优先算法。

解决方案1(Java)

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/**
 * 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) {
        Set<Integer> set = new HashSet();
        return DFS(root, k, set);
    }
    
    private boolean DFS(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 DFS(root.left, k, set) || DFS(root.right, k, set);
    }
}

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