Leetcode 333. Largest BST Subtree

Given a binary tree, find the largest subtree which is a Binary Search Tree (BST), where largest means subtree with largest number of nodes in it.

Note:
A subtree must include all of its descendants.
Here's an example:

    10
    / \
   5  15
  / \   \ 
 1   8   7

The Largest BST Subtree in this case is the highlighted one.
The return value is the subtree's size, which is 3.

 

思路： 根据Hint，可以递归调用98题 Validate Binary Search Tree.

1. 写一个子函数，递归的计算tree的大小。

2. 从root开始，检查是否为BST，如果是，输出treeSize(root)。如果不是return max(largestBSTSubtree(root.right), largestBSTSubtree(root.left))

 

 1 class Solution(object):
 2     def largestBSTSubtree(self, root):
 3         """
 4         :type root: TreeNode
 5         :rtype: int
 6         """
 7         if self.isValidBST(root):
 8             return self.treeSize(root)
 9         else:
10             return max(self.largestBSTSubtree(root.left), self.largestBSTSubtree(root.right))
11             
12         
13     def isValidBST(self, root):
14         """
15         :type root: TreeNode
16         :rtype: bool
17         """
18         maxInt = 2147483647
19         minInt = -2147483648
20         
21         return self.isBSTHelper(root, minInt, maxInt)
22         
23     def isBSTHelper(self, root, minVal, maxVal):
24         if not root:
25             return True
26         
27         if root.val <= maxVal and root.val >= minVal and self.isBSTHelper(root.left, minVal, root.val - 1) and self.isBSTHelper(root.right, root.val + 1, maxVal):
28             return True
29         else:
30             return False
31             
32     def treeSize(self, root):
33         if not root:
34             return 0
35         else:
36             return 1 + self.treeSize(root.left) + self.treeSize(root.right)
37

 

posted @ 2017-01-24 14:52  lettuan  阅读(169)  评论(0)  编辑  收藏  举报