Based on the special property of the tree, we can guarantee that the root node is the smallest node in the tree. We just have to recursively traverse the tree and find a node that is bigger than the root node but smaller than any existing node we have come across.

*- Yangshun*

```
class Solution(object):
def findSecondMinimumValue(self, root):
res = [float('inf')]
def traverse(node):
if not node:
return
if root.val < node.val < res[0]:
res[0] = node.val
traverse(node.left)
traverse(node.right)
traverse(root)
return -1 if res[0] == float('inf') else res[0]
```