[Leetcode] Binary tree--653. Two Sum IV

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

 Solution:

        #traverse the tree and using's hash to store visited node
        dic = {}
        def helperRecursive(root):
            if root is None:
                return False
            if (k - root.val) in dic:
                return True
            if root.val not in dic:
                dic[root.val] = 1
            return helperRecursive(root.left) or helperRecursive(root.right)

        return helperRecursive(root)