C++迭代实现前中后序遍历

#include <iostream>
#include <vector>
#include <stack>
using namespace std;
struct Treenode
{
    int val;
    Treenode* left;
    Treenode* right;
    Treenode(int _val):val(_val),left(nullptr),right(nullptr){}
};
vector<int> inorder(Treenode *root)
{
    //if(root==nullptr) return nullptr;
    vector<int> v;
    stack<Treenode*> st;
    st.push(root);
    while(!st.empty())
    {
        Treenode *node=st.top();
        if(node)
        {
            st.pop();
            //中序 
            if(node->right) st.push(node->right);//右节点入栈 
            st.push(node);//中间节点入栈 
            st.push(nullptr);//标志
            if(node->left) st.push(node->left); //左节点入栈
            //前序 
            //if(node->right) st.push(node->right);//右节点入栈 
            //if(node->left) st.push(node->left); //左节点入栈 
            //st.push(node);//中间节点入栈 
            //st.push(nullptr);//标志
            //后序
            //st.push(node);//中间节点入栈 
            //st.push(nullptr);//标志
            //if(node->right) st.push(node->right);//右节点入栈 
            //if(node->left) st.push(node->left); //左节点入栈 
            
        }
        else{//node为nullptr 
            st.pop();//nullptr出栈
            node=st.top();
            st.pop();//node保留的节点出栈 
            v.push_back(node->val);
        }
    }
    return v; 
} 
int main(int argc, char *argv[])
{
    Treenode* t1=new Treenode(1);
    Treenode* t2=new Treenode(2);
    Treenode* t3=new Treenode(4);
    Treenode* t4=new Treenode(5);
    Treenode* t5=new Treenode(6);
    t4->left=t3;
    t4->right=t5;
    t3->left=t1;
    t3->right=t2;
    vector<int> v;
    v=inorder(t4);
    for(auto it:v)
    {
        cout<<it<<" "; 
    }
     
    return 0;
}