题目
Given the root of a Binary Search Tree and a target number k, return true if there exist two elements in the BST such that their sum is equal to the given target.
解题思路
遍历这个树，并把遇到的数加到一个集合里，如果target-新遇到的数存在在集合里，则返回True
代码
class Solution:
def findTarget(self, root: Optional[TreeNode], k: int) -> bool:
nums = set()
s = [root]
while s:
cur = s.pop()
if k - cur.val in nums:
return True
nums.add(cur.val)
if cur.left:
s.append(cur.left)
if cur.right:
s.append(cur.right)
return False
