LeetCode: Validata 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 be binary search trees.

地址：

算法：中序遍历，看看遍历到的节点值是否递增。代码：

1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool isValidBST(TreeNode *root) {13         if(!root)   return true;14         stack
        
          stk;15         TreeNode *p = root;16         while(p){17             stk.push(p);18             p = p->left;19         }20         TreeNode *pre = NULL;21         p = NULL;22         while(!stk.empty()){23             pre = p;24             p = stk.top();25             stk.pop();26             if(pre && p->val <= pre->val){27                 return false;28             }29             if(p->right){30                 TreeNode *q = p->right;31                 while(q){32                     stk.push(q);33                     q = q->left;34                 }35             }36         }37         return true;38     }39 };