# Leetcode: Second Minimum Node In a Binary Tree

Second Minimum Node In a Binary Tree

Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly two or zero sub-node. If the node has two sub-nodes, then this node’s value is the smaller value among its two sub-nodes.

Given such a binary tree, you need to output the second minimum value in the set made of all the nodes’ value in the whole tree.

If no such second minimum value exists, output -1 instead.

```Example 1:
Input:
2
/ \
2   5
/ \
5   7

Output: 5
Explanation: The smallest value is 2, the second smallest value is 5.
```
```Example 2:
Input:
2
/ \
2   2

Output: -1
Explanation: The smallest value is 2, but there isn't any second smallest value.
```

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

```## Blog link: https://code.dennyzhang.com/second-minimum-node-in-a-binary-tree
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
def findSecondMinimumValue(self, root):
"""
:type root: TreeNode
:rtype: int
"""
## Idea: pre-order DFS
return self._findSecondMinimumValue(root, root.val)

def _findSecondMinimumValue(self, root, firstMininumValue):
if root is None:
return -1
if root.left is None and root.right is None:
if root.val > firstMininumValue:
return root.val
else:
return -1

if root.val > firstMininumValue:
return root.val
else:
left_ret, right_ret, ret = -1, -1, -1
if root.left:
left_ret = self._findSecondMinimumValue(root.left, firstMininumValue)
if root.right:
right_ret = self._findSecondMinimumValue(root.right, firstMininumValue)
if left_ret == -1 or right_ret == -1:
ret = max(left_ret, right_ret)
else:
ret = min(left_ret, right_ret)
return ret
```

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