Leetcode---98. Validate Binary Search Tree

Title Link: Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node ' s key.
• The right subtree of a node contains only nodes with keys greater than the node ' s key.
• Both the left and right subtrees must also is binary search trees.

Confused what "{1,#,2,3}" means? > read more about how binary tree was serialized on OJ.

OJ ' s Binary Tree serialization:

The serialization of a binary tree follows a level order traversal, where ' # ' signifies a path terminator where no node ex Ists below.

Here's an example:

   1  /  2   3    /   4         5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

The requirement of this question is to detect whether the binary tree is a two-fork search tree (BST).

Binary search tree, as the name implies, it is a binary tree, that is, each node under a maximum of 2 child nodes. In order to facilitate the search, the binary search tree is either an empty tree or a two-fork tree with the following properties:

• If the left subtree is not empty, the values of all nodes on the left subtree are smaller than the value of the root node;
• If the right subtree is not empty, the value of all nodes on the right subtree is greater than the value of the root node;
• The left and right sub-trees are also two-fork search trees respectively.

Binary search tree also has a feature is that the middle sequence traversal is strictly incremental, so you can use a feature to check whether a binary tree is a two-fork search tree. The pre-variable is used to record the previous node, then the two-fork tree is sequenced, and the number of the pre node is measured less than the current node.

Time complexity: O (N)

Space complexity: O (1)

1 class Solution2 {3  Public:4     BOOL Isvalidbst(TreeNode *Root)5     {6         TreeNode *Pre = NULL;7         Inordertraversal(Root, Pre);8     }9 Private:Ten     BOOL Inordertraversal(TreeNode *P, TreeNode *&Pre) One     { A         if(P == NULL) -             return true; -          the         if(!Inordertraversal(P  -  Left, Pre)) -             return false; -          -         if(Pre != NULL && Pre  - Val >= P  - Val) +             return false; -         Pre = P; +          A         if(!Inordertraversal(P  -  Right, Pre)) at             return false; -          -         return true; -     } - };

Reprint please indicate source: Leetcode---98. Validate Binary Search Tree

Leetcode---98. Validate Binary Search Tree