#include <iostream>
#include "queue"
#include "vector"
using namespace std;
struct TreeNode
{
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode* next;
    TreeNode(int value):val(value),left(nullptr),right(nullptr),next(nullptr)
    {
    }
};

vector<TreeNode*> level;
vector<TreeNode*> operateTree(TreeNode* root)
{
    if(!root) return level;
    queue<TreeNode*> q;
    q.push(root);
    
    while(!q.empty())
    {
        int size=q.size();
        for(int i=0;i<size;i++)
        {
            TreeNode *t=q.front();
            q.pop();
            if(i!=size-1)   
                t->next = q.front();
            level.push_back(t);
            if(t->left) 
            {
                q.push(t->left);
            }
            if(t->right)
            {
                q.push(t->right);
            }
        }
        
    }
    return level;
}

int main() {
    TreeNode *root=new TreeNode(0);
    root->left=new TreeNode(1);
    root->right=new TreeNode(2);
    root->left->left=new TreeNode(3);
    root->left->right=new TreeNode(4);
    root->right->left=new TreeNode(5);
    root->right->right=new TreeNode(6);
    operateTree(root);
    
    for(auto node:level)
    {
        if(node->next==nullptr)
        {
            cout<<"cur node: "<<node->val<<"->"<<"nullptr"<<endl;
        }
        else
        {
            cout<<"cur node: "<<node->val<<"->"<<node->next->val<<endl;
        }
    }
}