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;
    }
};