【刷题笔记】98. Validate Binary Search Tree

题目

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.

xample 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.

题目大意

给定一个二叉树，判断其是否是一个有效的二叉搜索树。假设一个二叉搜索树具有如下特征：

• 节点的左子树只包含小于当前节点的数。
• 节点的右子树只包含大于当前节点的数。
• 所有左子树和右子树自身必须也是二叉搜索树。

解题思路

• 判断一个树是否是 BST，按照定义递归判断即可

参考代码

package leetcode

import (
	"fmt"
	"testing"

	"github.com/halfrost/LeetCode-Go/structures"
)

type question98 struct {
	para98
	ans98
}

// para 是参数
// one 代表第一个参数
type para98 struct {
	one []int
}

// ans 是答案
// one 代表第一个答案
type ans98 struct {
	one bool
}

func Test_Problem98(t *testing.T) {

	qs := []question98{

		{
			para98{[]int{10, 5, 15, structures.NULL, structures.NULL, 6, 20}},
			ans98{false},
		},

		{
			para98{[]int{}},
			ans98{true},
		},

		{
			para98{[]int{2, 1, 3}},
			ans98{true},
		},

		{
			para98{[]int{5, 1, 4, structures.NULL, structures.NULL, 3, 6}},
			ans98{false},
		},
	}

	fmt.Printf("------------------------Leetcode Problem 98------------------------\n")

	for _, q := range qs {
		_, p := q.ans98, q.para98
		fmt.Printf("【input】:%v      ", p)
		rootOne := structures.Ints2TreeNode(p.one)
		fmt.Printf("【output】:%v      \n", isValidBST(rootOne))
	}
	fmt.Printf("\n\n\n")
}