【算法题】二叉树非递归遍历

先序遍历：入栈时访问
中序遍历：出栈时访问
后续遍历：第二次出栈时访问

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#include <iostream>#include <vector>#include <numeric>#include<algorithm>#include <stack>#include <string>using namespace std;struct Tree{    double value;    Tree* left;    Tree* right;};void PreOrder(Tree * root)//先序遍历{    stack<Tree*> s;    Tree* p = root;    while (p != NULL || !s.empty())    {        while (p != NULL)        {            cout << p->value<<" ";            s.push(p);            p = p->left;        }        if (!s.empty())        {            p = s.top();            s.pop();            p = p->right;        }    }}void MidOrder(Tree * root)//中序遍历 {    stack<Tree*> s;    Tree * p = root;    while (p != NULL || !s.empty())    {        while (p != NULL)        {            s.push(p);            p = p->left;        }        if (!s.empty())        {            p = s.top();            s.pop();            cout << p->value << " ";            p = p->right;        }    }}void PostOrder(Tree* root)//后序遍历{    struct Note    {        Tree * p;        bool isFirst;    };    stack<Note> s;    Note tmp;    Tree* p=root;    while (p != NULL || !s.empty())    {        while (p != NULL)        {            tmp.p = p;            tmp.isFirst = true;            s.push(tmp);            p = p->left;        }        if (!s.empty())        {            tmp = s.top();            s.pop();            if (tmp.isFirst == true)            {                p = tmp.p->right;                tmp.isFirst = false;                s.push(tmp);            }            else            {                cout << tmp.p->value << " ";                p = NULL;            }        }    }}int main(){    Tree root;    Tree node1_1;    Tree node1_2;    Tree node2_1;    Tree node2_2;    Tree node2_3;    Tree node3_1;    Tree node3_2;    Tree node3_3;    Tree node3_4;    root.value = 0;    node1_1.value = 1.1;    node1_2.value = 1.2;    node2_1.value = 2.1;    node2_2.value = 2.2;    node2_3.value = 2.3;    node3_1.value = 3.1;    node3_2.value = 3.2;    node3_3.value = 3.3;    node3_4.value = 3.4;    root.left = &node1_1;    root.right = &node1_2;    node1_1.left = &node2_1;    node1_1.right = &node2_2;    node1_2.left = &node2_3;    node1_2.right = NULL;    node2_1.left = &node3_1;    node2_1.right = &node3_2;    node2_2.left = NULL;    node2_2.right = NULL;    node2_3.left = &node3_3;    node2_3.right = &node3_4;    node3_1.left = NULL;    node3_1.right = NULL;    node3_2.left = NULL;    node3_2.right = NULL;    node3_3.left = NULL;    node3_3.right = NULL;    node3_4.left = NULL;    node3_4.right = NULL;    PreOrder(&root);    cout << endl;    MidOrder(&root);    cout << endl;    PostOrder(&root);    cout << endl;    return -1;}

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