653. Two Sum IV - Input is a BST

• Difficulty: Easy

• Topics: Tree

• Similar Questions:

  • Two Sum

  • Two Sum II - Input array is sorted

  • Two Sum III - Data structure design

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 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

Solutions:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        unordered_set<int> seen;
        return helper(root, k, seen);
    }

private:
    bool helper(TreeNode* root, int k, unordered_set<int>& seen) {
        if (root == nullptr)    return false;
        if (seen.count(k - root->val) > 0)  return true;
        seen.insert(root->val);
        return helper(root->left, k, seen) || helper(root->right, k, seen);
    }
};