文章标题

653 Two Sum IV - Input is a BST
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

想法是将排序树用中序遍历得到一个数组然后进行求和查找，看到
新的思路是在遍历过程中就进行查找。
代码：
def findTarget(self, root, k):
if not root:
return False
bfs, s = [root], set() # bfs = [root] ; s = set()
for i in bfs:
if k - i.val in s:
return True
s.add(i.val)
if i.left:
bfs.append(i.left)
if i.right:
bfs.append(i.right)
return False
在广度优先搜索的过程中不断刷新list和set