98. Validate Binary Search Tree &&面试题 04.05. Legal Binary Search Tree LCCI

Problem

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.

Example1

Example2

Solution

利用BST中序遍是递增序列的特性。

非递归，中序遍历BST。

/**
 * Definition for a binary tree node.
 * 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*> sk;
        TreeNode *cur = root;

        TreeNode *prev = NULL;

        while(cur || !sk.empty())
        {
            while(cur)
            {
                sk.push(cur);
                cur = cur->left;
            }

            cur = sk.top();
            sk.pop();

            if(prev)
            {
                if(prev->val >= cur->val)
                {
                    return false;
                }
                else
                {
                    prev = cur;
                }
            }
            else
            {
                prev = cur;
            }

            cur = cur->right;

        }

        return true;

    }
};