题目：
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
题解：
q,s=root and [root],set()
while q:
new=[]
for node in q:
if k-node.val in s:
return True
else:
s.add(node.val)
new.extend([child for child in [node.left,node.right] if child])
q=new
return False