Validate Binary Search Tree

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.

Example 1:

    2   /   1   3

Binary Tree [2,1,3] , return True.

Example 2:

    1   /   2   3

Binary tree [1,2,3] , return false.

Analysis: determine if the sequence traversal is gradually increasing, if not, return false, if yes, return true.

/** Definition for a binary tree node. * struct TreeNode {* int val; * TreeNode *left; * TreeNode *right; * TreeNode (int x): Val (x), left (null), right (NULL) {} *}; */classSolution { Public:    BOOLIsvalidbst (treenode*root) {        if(root==NULL)return true; Stack<TreeNode*>s; TreeNode* cur=Root; TreeNode* pre=nullptr;  while(1){                         while(cur) {s.push (cur); Cur=cur->Left ; }            if(S.empty ()) Break; Cur=S.top ();            S.pop (); if(pre!=nullptr && pre->val >= cur->val)return false; Pre=cur; Cur= cur->Right ; }                return true; }};

Validate Binary Search Tree