01_05_linked_list__reverse_knodes.cpp

#include<bits/stdc++.h>

using namespace std;


#define fastio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define MOD 1000000007
#define MOD1 998244353
#define INF 1e18
#define nline "\n"
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define ff first
#define ss second
#define PI 3.141592653589793238462
#define set_bits __builtin_popcountll
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()

typedef long long ll;
typedef unsigned long long ull;
typedef long double lld;
// typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update > pbds; // find_by_order, order_of_key

#ifndef ONLINE_JUDGE
#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
#else
#define debug(x)
#endif

void _print(ll t) {cerr << t;}
void _print(int t) {cerr << t;}
void _print(string t) {cerr << t;}
void _print(char t) {cerr << t;}
void _print(lld t) {cerr << t;}
void _print(double t) {cerr << t;}
void _print(ull t) {cerr << t;}

template <class T, class V> void _print(pair <T, V> p);
template <class T> void _print(vector <T> v);
template <class T> void _print(set <T> v);
template <class T, class V> void _print(map <T, V> v);
template <class T> void _print(multiset <T> v);
template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ff); cerr << ","; _print(p.ss); cerr << "}";}
template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}

bool comp(int a, int b)
{
    return (a < b);
}
// #define int long long
class node{
public:
    int data;
    node *next;
};
void insert_in_begning(node* &head_ref,int x)
{
    node *new_node=new node();
    new_node->data=x;
    new_node->next=head_ref;
    head_ref=new_node;
}

void insert_in_end(node *head_ref,int x)
{ 
    node*new_node=new node();
    node *temp=head_ref;
    new_node->next=NULL;
    new_node->data=x;
    if(head_ref==NULL)
    {
        head_ref=new_node;
        return;
    }
    while(temp->next!=NULL)
    {
        temp=temp->next;
    }
    temp->next=new_node;
    return;

}

//reversing the linked list itrative way 
node* reverse_itrative(node* head)
{
    node* curr=head;
    node* prev=NULL;
    node* nxt;
    while (curr!=NULL)
    {
        nxt=curr->next;
        curr->next=prev;

        prev=curr;
        curr=nxt;
    }
    return prev;
}

//reversing  the list recurssive way
node* reverse_recursive(node* &head)
{
     if(head==NULL || head->next==NULL) //it means head is empty or having just one element 
           return head;

       node* new_head=reverse_recursive(head->next); 
       head->next->next=head;
       head->next=NULL;
       return new_head;
}

//reversingthe list after every k elements
node* reverse_after_Knode(node* head,int k)
{
    node *prev=NULL;
    node *curr=head;
    node *nxt;
    int count =0;
   
    while( count<k && curr!=NULL)
    {
        nxt=curr->next;
        curr->next=prev;
    
       prev=curr;
       curr=nxt;   
       count++; 
    }
    if(curr!=NULL)
      head->next=reverse_after_Knode(curr,k);
     
     return prev;
}

 void print_list(node *temp)
{
    while(temp!=NULL)
    {
    cout<<temp->data<<"->";
        temp=temp->next;
    }cout<<"NULL"<<nline;
}

void insert_after(node *temp,int x)
{
    node *new_node=new node;
    new_node->next=NULL;
    new_node->data=x;
   if(temp==NULL)
   {
    cout<<"node can not be null";
     return;
   }
    new_node->next=temp->next;
    temp->next=new_node;
    return;
}
void solve(){
    // int n;
    // cin>>n;
    node *head=NULL;
 
  insert_in_begning(head,2);
  insert_in_begning(head,1);
  insert_in_end(head,3);
  insert_in_end(head,4);
  insert_in_end(head,5);
  insert_in_end(head,6);
  insert_in_end(head,7);
  insert_in_end(head,8);
    print_list(head);
     // node* new_head2= reverse_recursive(head);
     // print_list(new_head2);
     // node* new_head1= reverse_itrative(head);
     // print_list(new_head1);
     node* new_head3=reverse_after_Knode(head,2);
     print_list(new_head3);


}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    freopen("Error.txt", "w", stderr);
    #endif // ONLINE_JUDGE

    // int t;
    // cin >> t;
    // while(t--){
     solve();
    //  cout<<endl;
    // }
    return 0;
}