LeetCode: Validata Binary Search Tree

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：https://oj.leetcode.com/problems/validate-binary-search-tree/

算法：中序遍历，看看遍历到的节点值是否递增。代码：

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     bool isValidBST(TreeNode *root) {
         if(!root)   return true;
         stack<TreeNode*> stk;
         TreeNode *p = root;
         while(p){
             stk.push(p);
             p = p->left;
         }
         TreeNode *pre = NULL;
         p = NULL;
         while(!stk.empty()){
             pre = p;
             p = stk.top();
             stk.pop();
             if(pre && p->val <= pre->val){
                 return false;
             }
             if(p->right){
                 TreeNode *q = p->right;
                 while(q){
                     stk.push(q);
                     q = q->left;
                 }
             }
         }
         return true;
     }
 };