本质上是递归遍历左右后在与根节点做判断，本质上是后序遍历

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

给你一个二叉树，判断他是否是个有效的二叉查找树(BST)。

假定一个BST树按照下面的内容定义：

  • 左子树的节点的值都小于父节点。
  • 右子树的节点的值都大于父节点。
  • 左子树和右子树都得是合法的而二叉查找树。

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

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.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
test.cpp：
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#include <iostream>
#include <cstdio>
#include <stack>
#include <vector>
#include "BinaryTree.h"

using namespace std;

/**
 * Definition for binary tree
 * struct TreeNode {
 * int val;
 * TreeNode *left;
 * TreeNode *right;
 * TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
bool islessthanroot(TreeNode *left, int val)
{
    if(left == NULL)
    {
        return true;
    }
    if(left->val >= val)
    {
        return false;
    }
    return islessthanroot(left->left, val) && islessthanroot(left->right, val);
}

bool islargethanroot(TreeNode *right, int val)
{
    if(right == NULL)
    {
        return true;
    }
    if(right->val <= val)
    {
        return false;
    }
    return islargethanroot(right->left, val) && islargethanroot(right->right, val);
}

bool isValidBST(TreeNode *root)
{
    if(root == NULL)
    {
        return true;
    }
    if(islessthanroot(root->left, root->val) && islargethanroot(root->right, root->val))
    {
        return isValidBST(root->left) && isValidBST(root->right);
    }
    else
    {
        return false;
    }
}

// 树中结点含有分叉，
//                  6
//              /       \
//             2         7
//           /   \
//          1     4
//               / \
//              3   5
int main()
{
    TreeNode *pNodeA1 = CreateBinaryTreeNode(6);
    TreeNode *pNodeA2 = CreateBinaryTreeNode(2);
    TreeNode *pNodeA3 = CreateBinaryTreeNode(7);
    TreeNode *pNodeA4 = CreateBinaryTreeNode(1);
    TreeNode *pNodeA5 = CreateBinaryTreeNode(4);
    TreeNode *pNodeA6 = CreateBinaryTreeNode(3);
    TreeNode *pNodeA7 = CreateBinaryTreeNode(5);

ConnectTreeNodes(pNodeA1, pNodeA2, pNodeA3);
    ConnectTreeNodes(pNodeA2, pNodeA4, pNodeA5);
    ConnectTreeNodes(pNodeA5, pNodeA6, pNodeA7);

bool ans = isValidBST(pNodeA1);

if (ans == true)
    {
        cout << "Valid BST" << endl;
    }
    else
    {
        cout << "Not Valid BST" << endl;
    }

DestroyTree(pNodeA1);
    return 0;
}

 


结果输出：


Valid BST





BinaryTree.h:












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#ifndef _BINARY_TREE_H_
#define _BINARY_TREE_H_


struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};


TreeNode *CreateBinaryTreeNode(int value);
void ConnectTreeNodes(TreeNode *pParent,
                      TreeNode *pLeft, TreeNode *pRight);
void PrintTreeNode(TreeNode *pNode);
void PrintTree(TreeNode *pRoot);
void DestroyTree(TreeNode *pRoot);


#endif /*_BINARY_TREE_H_*/












BinaryTree.cpp:










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#include <iostream>
#include <cstdio>
#include "BinaryTree.h"


using namespace std;


/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */


//创建结点
TreeNode *CreateBinaryTreeNode(int value)
{
    TreeNode *pNode = new TreeNode(value);


return pNode;
}


//连接结点
void ConnectTreeNodes(TreeNode *pParent, TreeNode *pLeft, TreeNode *pRight)
{
    if(pParent != NULL)
    {
        pParent->left = pLeft;
        pParent->right = pRight;
    }
}


//打印节点内容以及左右子结点内容
void PrintTreeNode(TreeNode *pNode)
{
    if(pNode != NULL)
    {
        printf("value of this node is: %d\n", pNode->val);


if(pNode->left != NULL)
            printf("value of its left child is: %d.\n", pNode->left->val);
        else
            printf("left child is null.\n");


if(pNode->right != NULL)
            printf("value of its right child is: %d.\n", pNode->right->val);
        else
            printf("right child is null.\n");
    }
    else
    {
        printf("this node is null.\n");
    }


printf("\n");
}


//前序遍历递归方法打印结点内容
void PrintTree(TreeNode *pRoot)
{
    PrintTreeNode(pRoot);


if(pRoot != NULL)
    {
        if(pRoot->left != NULL)
            PrintTree(pRoot->left);


if(pRoot->right != NULL)
            PrintTree(pRoot->right);
    }
}


void DestroyTree(TreeNode *pRoot)
{
    if(pRoot != NULL)
    {
        TreeNode *pLeft = pRoot->left;
        TreeNode *pRight = pRoot->right;


delete pRoot;
        pRoot = NULL;


DestroyTree(pLeft);
        DestroyTree(pRight);
    }
}












 


 


 


												


											
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