98. Validate Binary Search Tree

这是leetcode邮件推送的题目，bug-free的代码还是需要debug一段时间的

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.

Example 1:

Input:
    2
   / 
  1   3
Output: true

Example 2:

    5
   / 
  1   4
     / 
    3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
             is 5 but its right child's value is 4.

注意指针的引用 TreeNode* &pre

/**
 * 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 {
private:
    bool validate(TreeNode* root,TreeNode* &pre){
        if(root==nullptr)
            return true;
        //cout<<"root->val: "<<root->val<<endl;
        if(!validate(root->left,pre))
            return false;
        if(pre!=nullptr && (pre->val>=root->val))
            return false;
        pre=root; 
        
        return validate(root->right,pre);
        
    }
public:
    bool isValidBST(TreeNode* root) {
        TreeNode* pre=nullptr;
        return validate(root,pre); 
    }
};

python代码

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def isValidBST(self, root, largerThan=float('-inf'), lessThan=float('inf')):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True
        if root.val <= largerThan or root.val >= lessThan:
            return False

        return self.isValidBST( root.left, largerThan, min(lessThan, root.val)) and 
               self.isValidBST(root.right, max(largerThan, root.val), lessThan)