Validate Binary Search Tree - Leetcode

最新推荐文章于 2019-03-18 16:47:00 发布

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本文通过中序遍历的方式，阐述了如何检查给定的二叉树是否符合二叉搜索树（BST）的定义，即左子树上所有节点的值小于根节点的值，右子树上所有节点的值大于根节点的值。

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/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {  
        Stack<TreeNode> s = new Stack<>();
        ArrayList<Integer> ai = new ArrayList<>();
        TreeNode pointer = root;
        while(pointer!=null || !s.empty()){
            if(pointer!=null){
                s.push(pointer);
                pointer=pointer.left;
            }else{
                int pre = s.peek().val;
                if(ai.size()>=1 && pre<=ai.get(ai.size()-1))
                   return false;
                ai.add(pre);
                pointer=s.pop().right;
            }
        }
        return true;
      }  
}

中序遍历一棵树，看看是否有打破递增的规律。

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.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

OJ's Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".