文件一：main.cpp

// 面试题：二叉树的下一个结点
// 题目：给定一棵二叉树和其中的一个结点，如何找出中序遍历顺序的下一个结点？
// 树中的结点除了有两个分别指向左右子结点的指针以外，还有一个指向父结点的指针。

#include <iostream>
#include "BinaryTree.h"
using namespace std;

BinaryTreeNode* GetNext(BinaryTreeNode* pNode)
{
    if (pNode == NULL)
        return NULL;

    BinaryTreeNode* pNext = NULL;//跟占位符一样的pNext
    if (pNode->m_pRight != NULL)//第一种情况：我有右孩子
    {
        pNext = pNode->m_pRight;//从这个右孩子开始，一直向下找到没有左孩子为止，则这个节点就是pNext
        while (pNext->m_pLeft != NULL)
            pNext = pNext->m_pLeft;
    }
    else if (pNode->m_pParent != NULL)//第二种情况：我没有右孩子，而我有父节点
    {
        BinaryTreeNode* pCurrent = pNode;
        pNext = pNode->m_pParent;
        while (pNext != NULL && pCurrent == pNext->m_pRight)//从我这个开始向上找父节点，一直找到我这一脉是该父节点的左孩子，如果遇到根节点还没有那就没了
        {
            pCurrent = pNext;
            pNext = pNext->m_pParent;
        }
    }

    return pNext;
}

// ====================测试代码====================
void Test(const char* testName, BinaryTreeNode* pNode, BinaryTreeNode* expected)
{
    if (testName != NULL)
        printf("%s begins: ", testName);

    BinaryTreeNode* pNext = GetNext(pNode);
    if (pNext == expected)
        printf("Passed.\n");
    else
        printf("FAILED.\n");
}

//            8
//        6      10
//       5 7    9  11
void Test1_7()
{
    BinaryTreeNode* pNode8 = CreateBinaryTreeNode(8);
    BinaryTreeNode* pNode6 = CreateBinaryTreeNode(6);
    BinaryTreeNode* pNode10 = CreateBinaryTreeNode(10);
    BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);
    BinaryTreeNode* pNode7 = CreateBinaryTreeNode(7);
    BinaryTreeNode* pNode9 = CreateBinaryTreeNode(9);
    BinaryTreeNode* pNode11 = CreateBinaryTreeNode(11);

    ConnectTreeNodes(pNode8, pNode6, pNode10);
    ConnectTreeNodes(pNode6, pNode5, pNode7);
    ConnectTreeNodes(pNode10, pNode9, pNode11);

    Test("Test1", pNode8, pNode9);
    Test("Test2", pNode6, pNode7);
    Test("Test3", pNode10, pNode11);
    Test("Test4", pNode5, pNode6);
    Test("Test5", pNode7, pNode8);
    Test("Test6", pNode9, pNode10);
    Test("Test7", pNode11, NULL);

    DestroyTree(pNode8);
}

//            5
//          4
//        3
//      2
void Test8_11()
{
    BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);
    BinaryTreeNode* pNode4 = CreateBinaryTreeNode(4);
    BinaryTreeNode* pNode3 = CreateBinaryTreeNode(3);
    BinaryTreeNode* pNode2 = CreateBinaryTreeNode(2);

    ConnectTreeNodes(pNode5, pNode4, NULL);
    ConnectTreeNodes(pNode4, pNode3, NULL);
    ConnectTreeNodes(pNode3, pNode2, NULL);

    Test("Test8", pNode5, NULL);
    Test("Test9", pNode4, pNode5);
    Test("Test10", pNode3, pNode4);
    Test("Test11", pNode2, pNode3);

    DestroyTree(pNode5);
}

//        2
//         3
//          4
//           5
void Test12_15()
{
    BinaryTreeNode* pNode2 = CreateBinaryTreeNode(2);
    BinaryTreeNode* pNode3 = CreateBinaryTreeNode(3);
    BinaryTreeNode* pNode4 = CreateBinaryTreeNode(4);
    BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);

    ConnectTreeNodes(pNode2, NULL, pNode3);
    ConnectTreeNodes(pNode3, NULL, pNode4);
    ConnectTreeNodes(pNode4, NULL, pNode5);

    Test("Test12", pNode5, NULL);
    Test("Test13", pNode4, pNode5);
    Test("Test14", pNode3, pNode4);
    Test("Test15", pNode2, pNode3);

    DestroyTree(pNode2);
}

void Test16_17()
{
    BinaryTreeNode* pNode5 = CreateBinaryTreeNode(5);

    Test("Test16", pNode5, NULL);
    Test("Test17", NULL, NULL);

    DestroyTree(pNode5);
}

int main(int argc, char* argv[])
{
    Test1_7();
    Test8_11();
    Test12_15();
    Test16_17();

    system("pause");
}

文件二：BinaryTree.h

#ifndef BINARY_TREE_H
#define BINARY_TREE_H

struct BinaryTreeNode
{
    int                    m_nValue;
    BinaryTreeNode*        m_pLeft;
    BinaryTreeNode*        m_pRight;
    BinaryTreeNode*        m_pParent;
};

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

#endif

文件三：BinaryTree.cpp

#include <iostream>
#include "BinaryTree.h"
using namespace std;

BinaryTreeNode* CreateBinaryTreeNode(int value)//还是建立新节点
{
    BinaryTreeNode* pNode = new BinaryTreeNode();
    pNode->m_nValue = value;
    pNode->m_pLeft = NULL;
    pNode->m_pRight = NULL;
    pNode->m_pParent = NULL;

    return pNode;
}

void ConnectTreeNodes(BinaryTreeNode* pParent, BinaryTreeNode* pLeft, BinaryTreeNode* pRight)//一个父节点连接左右两孩子
{
    if (pParent != NULL)
    {
        pParent->m_pLeft = pLeft;
        pParent->m_pRight = pRight;

        if (pLeft != NULL)
            pLeft->m_pParent = pParent;
        if (pRight != NULL)
            pRight->m_pParent = pParent;
    }
}

void PrintTreeNode(BinaryTreeNode* pNode)//打印当前节点
{
    if (pNode != NULL)
    {
        cout << "value of this node is:" << pNode->m_nValue << endl;

        if (pNode->m_pLeft != NULL)
            cout << "value of its left child is:" << pNode->m_pLeft->m_nValue << endl;
        else
            cout << "left child is null.\n";

        if (pNode->m_pRight != NULL)
            cout << "value of its right child is: " << pNode->m_pRight->m_nValue << endl;
        else
            cout << "right child is null.\n";
    }
    else
    {
        cout << "this node is null.\n";
    }

    cout << endl;
}

void PrintTree(BinaryTreeNode* pRoot)//打印整个树
{
    PrintTreeNode(pRoot);

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

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

void DestroyTree(BinaryTreeNode* pRoot)//删除整个树
{
    if (pRoot != NULL)
    {
        BinaryTreeNode* pLeft = pRoot->m_pLeft;
        BinaryTreeNode* pRight = pRoot->m_pRight;

        delete pRoot;
        pRoot = NULL;

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