二叉树的三种遍历的递归与非递归算法

今天复习了一下二叉树的前序遍历、中序遍历、后序遍历的递归与非递归算法，顺便记录一下：

//TreeTest.h
#include <iostream>
struct TreeNode
{
    int value;
    TreeNode* leftChild;
    TreeNode* rightChild;
    void print()
    {
        printf("%d ",value);
    }

};
class MyTree
{
private:
    TreeNode* m_root;
    TreeNode* create(TreeNode*& root);
    void destroy(TreeNode* root);
    void preOrderRecursive(TreeNode* root);
    void inOrderRecursive(TreeNode* root);
    void postOrderRecursive(TreeNode* root);
    void levelOrderRecursive(TreeNode* root);
public:
    MyTree();
    ~MyTree();
    TreeNode* getTreeNode(int value);
    void createTreeForPreOrder();
    void preOrderRecursive();
    void inOrderRecursive();
    void postOrderRecursive();
    void levelOrderRecursive();

    //非递归遍历
    void preOrderUnRecursive();
    void inOrderUnRecursive();
    void postOrderUnRecursive();
};

//TreeTest.cpp
#include "TreeTest.h"
#include <iostream>
#include <queue>
#include <stack>
using namespace std;

MyTree::MyTree() : m_root(NULL)
{
}
MyTree::~MyTree()
{
    this->destroy(m_root);
    m_root = NULL;
}
void MyTree::destroy(TreeNode* root)
{
    if (!root) return;
    TreeNode* left=NULL, *right=NULL;

    left = root->leftChild;
    right = root->rightChild;
    delete root;

    if (left) destroy(left);
    if (right) destroy(right);
}

TreeNode* MyTree::create(TreeNode*& root)
{
    int value;
    cin>>value;
    if (value <= 0) return NULL;  //小于或等于0代表NULL
    if (!root) 
       root = this->getTreeNode(value);
    if (!root->leftChild)
        this->create(root->leftChild);
    if (!root->rightChild)
        this->create(root->rightChild);
    return root;
}

TreeNode* MyTree::getTreeNode(int value)
{
    TreeNode* p = new TreeNode();
    p->value = value;
    p->leftChild = p->rightChild = NULL;
    return p;
}

void MyTree::createTreeForPreOrder()
{
    this->create(m_root);
}

void MyTree::preOrderRecursive(TreeNode* root)
{
    if (!root) return;
    printf("%d ",root->value);
    if (root->leftChild) this->preOrderRecursive(root->leftChild);
    if (root->rightChild) this->preOrderRecursive(root->rightChild);
}

void MyTree::inOrderRecursive(TreeNode* root)
{
    if (!root) return;
    if (root->leftChild) this->inOrderRecursive(root->leftChild);
    printf("%d ",root->value);
    if (root->rightChild) this->inOrderRecursive(root->rightChild);
}

void MyTree::postOrderRecursive(TreeNode* root)
{
    if (!root) return;
    if (root->leftChild) this->postOrderRecursive(root->leftChild);
    if (root->rightChild) this->postOrderRecursive(root->rightChild);
    printf("%d ", root->value);
}

void MyTree::levelOrderRecursive(TreeNode* root)
{
    queue<TreeNode*> Q;
    if (root) Q.push(root);
    while (!Q.empty())
    {
        TreeNode* node = Q.front();
        Q.pop();
        node->print();
        if (node->leftChild) Q.push(node->leftChild);
        if (node->rightChild) Q.push(node->rightChild);
    }
}

void MyTree::preOrderRecursive()
{
    printf("递归前序遍历：");
    this->preOrderRecursive(m_root);
    printf("\n");
}
void MyTree::inOrderRecursive()
{
    printf("递归中序遍历：");
    this->inOrderRecursive(m_root);
    printf("\n");
}
void MyTree::postOrderRecursive()
{
    printf("递归后序遍历：");
    this->postOrderRecursive(m_root);
    printf("\n");
}
void MyTree::levelOrderRecursive()
{
    printf("层序遍历：");
    this->levelOrderRecursive(m_root);
    printf("\n");
}

void MyTree::preOrderUnRecursive()
{
    if (!m_root) return;
    printf("非递归先序遍历：");
    stack<TreeNode*> S;
    S.push(m_root);
    while (!S.empty())
    {
        TreeNode* node = S.top();
        S.pop();
        node->print();
        if (node->rightChild) S.push(node->rightChild);
        if (node->leftChild) S.push(node->leftChild);
    }
    printf("\n");
}
void MyTree::inOrderUnRecursive()
{
    if (!m_root) return;
    printf("非递归中序遍历：");
    stack<TreeNode*> S;
    TreeNode* p = m_root;
    
    while (p || !S.empty())
    {
        while (p)
        {
            S.push(p);
            p = p->leftChild;
        }

        p = S.top();
        S.pop();
        p->print();

        /*由于是中序遍历，那么访问了当前节点后，
        下一步必定要访问该节点的右子树*/
        p = p->rightChild;
    }
    printf("\n");
}
void MyTree::postOrderUnRecursive()
{
    if (!m_root) return;
    printf("非递归后序遍历：");
    stack<TreeNode*> S;
    TreeNode* p = m_root;
    TreeNode* q = NULL;
    while (p || !S.empty())
    {
        while (p)
        {
            S.push(p);
            p = p->leftChild;
        }

        p = S.top();

        /*
        判断表示当前节点的右节点没有或已经访问过
        q表示上一次访问的节点，由于是后续遍历，
        若当前节点有右节点，那么上一次访问的节点一定是当前节点的右节点
        */
        if (!p->rightChild || p->rightChild == q)   
        {
            p->print();
            q = p;
            p = NULL;
            S.pop();
        }
        else
        {
            p = p->rightChild;
        }
    }
    printf("\n");
}
//main.cpp
#include "TreeTest.h"
int main()
{
    MyTree tree;

    tree.createTreeForPreOrder();
    tree.preOrderRecursive();
    tree.preOrderUnRecursive();
    tree.inOrderRecursive();
    tree.inOrderUnRecursive();
    tree.postOrderRecursive();
    tree.postOrderUnRecursive();
    tree.levelOrderRecursive();

    getchar();
    getchar();
    return 0;
}

测试用例：

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