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")
}
