练习题

重建二叉树
// 题目：输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。
#include <iostream>
#include <exception>

//二叉树结构体
struct BinaryTreeNode{
    int value;//根
    BinaryTreeNode *pLeft;//左
    BinaryTreeNode *pRight;//右
};

//打印二叉树
void PrintTreeNode(const BinaryTreeNode*pNode)
{
    if (pNode != nullptr)
    {
        printf("value of this node is : %d
", pNode->value);

        if (pNode->pLeft != nullptr)
        {
            printf("value of its left child is : %d
", pNode->pLeft->value);
        }
        else{
            printf("left child is nullptr...
");
        }

        if (pNode->pRight != nullptr)
        {
            printf("value of its right child is : %d
", pNode->pRight->value);
        }
        else{
            printf("right child is nullptr...
");
        }
    }
    else{
        printf("this node is nullptr...
");
    }
    printf("
");
}
void PrintTree(const BinaryTreeNode* pRoot)
{
    PrintTreeNode(pRoot);

    if (pRoot != nullptr)
    {
        if (pRoot->pLeft != nullptr)
            PrintTree(pRoot->pLeft);

        if (pRoot->pRight != nullptr)
            PrintTree(pRoot->pRight);
    }
}

//销毁二叉树
void DestroyTree(BinaryTreeNode* pRoot)
{
    if (pRoot != nullptr)
    {
        BinaryTreeNode* pLeft = pRoot->pLeft;
        BinaryTreeNode* pRight = pRoot->pRight;

        delete pRoot;
        pRoot = nullptr;

        DestroyTree(pLeft);
        DestroyTree(pRight);
    }
}
BinaryTreeNode* ConstructCore(int* startPreorder, int* endPreorder, int* startInorder, int* endInorder)
{
    int rootValue = startPreorder[0];//前序遍历第一个节点是根节点
    BinaryTreeNode* root = new BinaryTreeNode();
    root->value = rootValue;
    root->pLeft = nullptr;
    root->pRight = nullptr;

    if (startPreorder == endPreorder) //只有一个根节点
    {
        if ((startInorder == endInorder) && (startPreorder[0] == startInorder[0]))
        {
            return root;
        }
        else{
            throw std::exception("Invalid input...111111");
        }
    }

    //从中序遍历中寻找根节点，左边为根节点的左子树，右边为根节点的右子树
    int* rootInorde = startInorder;
    while (rootInorde <= endInorder && (*rootInorde != rootValue))
    {
        rootInorde++;
    }
    if (rootInorde == endInorder && (*rootInorde != rootValue))//中序遍历完，没有找到根节点的值
    {
        throw std::exception("Invalid input...222222");
    }

    int leftLength = rootInorde - startInorder;//中序遍历中，根节点左边有多少个元素（左子树）
    int* leftPreorderEnd = startPreorder + leftLength; 
    if (leftLength > 0)
    {
        root->pLeft = ConstructCore(startPreorder + 1, leftPreorderEnd, startInorder, rootInorde - 1);
    }
    if (leftLength < endPreorder - startPreorder)
    {
        root->pRight = ConstructCore(leftPreorderEnd + 1, endPreorder, rootInorde + 1, endInorder);
    }
    return root;
}



// ============================测试代码==========================
int Test(char* testName, int*preorder, int*inorder, int length)
{
    if (preorder == nullptr || inorder == nullptr || length <= 0)
    {
        std::cout << "input error..." << std::endl;
        return 1;
    }

    if (testName != nullptr)
    {
        std::cout << testName << " begins:
";
    }
    std::cout << "The preorder sequence is:" << std::endl;
    for (int i = 0; i < length; i++)
    {
        std::cout << preorder[i] << " ";
    }
    std::cout << std::endl;

    std::cout << "The inorder sequence is:" << std::endl;
    for (int i = 0; i < length; i++)
    {
        std::cout << inorder[i] << " ";
    }
    std::cout << std::endl;

    //调用重建函数
    BinaryTreeNode* root = ConstructCore(preorder, preorder + length - 1, inorder, inorder + length - 1);
    PrintTree(root);
    DestroyTree(root);
    return 0;
}




void Test1()
{
    const int length = 8;
    int preorder[length] = { 1, 2, 4, 7, 3, 5, 6, 8 };
    int inorder[length] = { 4, 7, 2, 1, 5, 3, 8, 6 };

    Test("Test1", preorder, inorder, length);
}
void Test2()
{
    const int length = 5;
    int preorder[length] = { 1, 2, 3, 4, 5 };
    int inorder[length] = { 5, 4, 3, 2, 1 };

    Test("Test2", preorder, inorder, length);
}
int main()
{
    Test1();
}

从尾到头打印链表

#include <iostream>
#include <stack>
using namespace std;

struct ListNode{
    int value;
    struct ListNode* pNext;
};

void PrintListReversinglyIteratively(ListNode* pHead)
{
    stack<ListNode*> nodes;
    ListNode* pNode = pHead;
    while (pNode != nullptr)
    {
        nodes.push(pNode);
        pNode = pNode->pNext;
    }

    //输出栈
    while (!nodes.empty())
    {
        pNode = nodes.top();//取出栈顶元素
        cout << "value=" << pNode->value << ", ";
        nodes.pop();//弹出栈顶元素
    }
}

//方法二：递归法
void PrintListReversinglyRecursively(ListNode* pHead)
{
    if (pHead != nullptr)
    {
        if (pHead->pNext != nullptr)
        {
            PrintListReversinglyRecursively(pHead->pNext);
        }
        cout << "value=" << pHead->value << ",";
    }
}

//打印链表
void PrintList(ListNode* pHead)
{
    cout << "print list starts...
" << endl;

    ListNode* pNode = pHead;
    while (pNode != nullptr)
    {
        cout << "print list value=" << pNode->value << endl;
        pNode = pNode->pNext;
    }
    cout << "print list ends...
" << endl;
}

ListNode* CreateListNode(int value)
{
    ListNode* pNode = new ListNode();
    pNode->value = value;
    pNode->pNext = nullptr;
    return pNode;
}

void ConnectListNodes(ListNode* pCurrent, ListNode* pNext)
{
    if (pCurrent == nullptr)
    {
        cout << "error to connect two nodes" << endl;
        exit(-1);
    }
    pCurrent->pNext = pNext;
}

void DestroyList(ListNode* pHead)
{
    ListNode* pNode = pHead;
    while (pNode != nullptr)
    {
        pHead = pHead->pNext;
        delete pNode;
        pNode = pHead;
    }
}

// ====================测试代码====================
void Test(ListNode* pHead)
{
    PrintList(pHead);
    PrintListReversinglyIteratively(pHead);
    printf("

");
    PrintListReversinglyRecursively(pHead);
}

void Test1()
{
    cout << endl << "Test1 begins..." << endl;
    ListNode* pNode1 = CreateListNode(1);
    ListNode* pNode2 = CreateListNode(2);
    ListNode* pNode3 = CreateListNode(3);
    ListNode* pNode4 = CreateListNode(4);
    ListNode* pNode5 = CreateListNode(5);

    ConnectListNodes(pNode1, pNode2);
    ConnectListNodes(pNode2, pNode3);
    ConnectListNodes(pNode3, pNode4);
    ConnectListNodes(pNode4, pNode5);

    Test(pNode1);
    DestroyList(pNode1);
}

int main()
{
    Test1();
    return 0;
}