'res' always have 2 items at most.

```
class Solution:
def bfs(self, root, res):
if root:
if len(res) == 1:
if res[0] < root.val:
res.append(root.val)
else:
if res[1] > root.val and res[0] != root.val:
del res[1]
res.append(root.val)
self.bfs(root.left, res)
self.bfs(root.right, res)
def findSecondMinimumValue(self, root):
res = [root.val]
self.bfs(root, res)
return res[1] if len(res) > 1 else -1
```