题目描述

题目难度：Medium

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.

AC代码1

利用BST的性质：
对一个BST进行中序遍历，其结果是递增序列。

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */
class Solution {
    public boolean isValidBST(TreeNode root) {
        if(root == null) return true;
        List<Integer> list = new ArrayList<>();
        helper(root, list);
        return isSorted(list);
    }
    
    private void helper(TreeNode root, List<Integer> list){
        if(root == null) return;
        helper(root.left, list);
        list.add(root.val);
        helper(root.right, list);
    }
    
    private boolean isSorted(List<Integer> list){
        for(int i = 1;i < list.size();i++)
            if(list.get(i) <= list.get(i - 1)){
                return false;
            }
        return true;
    }
}

AC代码2

public class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    
    public boolean isValidBST(TreeNode root, long minVal, long maxVal) {
        if (root == null) return true;
        if (root.val >= maxVal || root.val <= minVal) return false;
        return isValidBST(root.left, minVal, root.val) && isValidBST(root.right, root.val, maxVal);
    }
}