98. Validate Binary Search Tree-蒲公英云

98. Validate Binary Search Tree

Bertha 。 2023-07-03 02:48 142阅读 0赞

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:

2
   / \
  1   3
Input: [2,1,3]
Output: true

Example 2:

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

题意：验证二叉搜索树。二叉搜索树满足：左子树上的值都小于节点值，右子树上的值都大于节点值，

解法一：中序遍历，结果递增，返回true，否则返回false。

/**
 * 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) {
        stack<TreeNode*> log;
        bool isFistValue = true;
        int preValue;
        while(root!=nullptr || !log.empty())
        {
            while(root)
            {
                log.push(root);
                root = root->left;
            }
            TreeNode* node = log.top();
            log.pop();   
            root = node->right;
            if(!isFistValue && preValue >= node->val)
            {
                return false;
            }
            isFistValue = false;
            preValue = node->val;
        }
        return true;
    }
};

方法二：递归。采用弱类型语言js，初始的min和max传入null。

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
 var Helper = function(root, min, max) 
 {
    if(root == null)
        return true;
    if(min >= root.val && min!=null)
        return false;
    if(max <= root.val && max!==null)
        return false;
    return Helper(root.left, min, root.val) && Helper(root.right, root.val, max);
};
var isValidBST = function(root) {
     return Helper(root, null, null);
};