lintcode: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 be binary search trees.
A single node tree is a BST

​​http://www.lintcode.com/en/problem/validate-binary-search-tree/#​​

1.需要两个值maxValue, minValue 来记录当前节点应该的取值范围。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    bool helper(TreeNode *node,long minVal,long maxVal){
        if(node==NULL){
            return true;
        }

        if(node->val<=minVal || node->val>=maxVal){
            return false;
        }

        return helper(node->left,minVal,node->val) & helper(node->right,node->val,maxVal);
    }

    bool isValidBST(TreeNode *root) {
        // write your code here
        return helper(root,LONG_MIN,LONG_MAX);
    }
};

2.判断中序遍历是否有序

这里写代码片/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    bool helper(TreeNode *root, long & pre){

        if(root==NULL){
            return true;
        }

        if(helper(root->left,pre)==false){
            return false;
        }

        if(root->val<=pre){
            return false;
        }else{
            pre=root->val;
        }

        if(helper(root->right,pre)==false){
            return false;
        }

        return true;
    }

    bool isValidBST(TreeNode *root) {
        // write your code here

        long pre=LONG_MIN;

        return helper(root,pre);
    }
};

参考：
​​​http://www.acmerblog.com/validate-binary-search-treeleetcode-6194.html​​​