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C++ Program For Making Middle Node Head In A Linked List

  • Last Updated : 13 Jan, 2022

Given a singly linked list, find middle of the linked list and set middle node of the linked list at beginning of the linked list. 
Examples:

Input: 1 2 3 4 5 
Output: 3 1 2 4 5

Input: 1 2 3 4 5 6
Output: 4 1 2 3 5 6 

https://media.geeksforgeeks.org/wp-content/uploads/Capturedsdw.png

The idea is to first find middle of a linked list using two pointers, first one moves one at a time and second one moves two at a time. When second pointer reaches end, first reaches middle. We also keep track of previous of first pointer so that we can remove middle node from its current position and can make it head.

C++




// C++ program to make middle node 
// as head of linked list. 
#include <bits/stdc++.h>
using namespace std;
  
// Link list node 
class Node 
    public:
    int data; 
    Node* next; 
}; 
  
/* Function to get the middle and set at 
   beginning of the linked list*/
void setMiddleHead(Node** head) 
    if (*head == NULL) 
        return
  
    // To traverse list nodes one by one 
    Node* one_node = (*head); 
  
    // To traverse list nodes by skipping 
    // one. 
    Node* two_node = (*head); 
  
    // To keep track of previous of middle 
    Node* prev = NULL; 
    while (two_node != NULL && two_node->next != NULL) 
    
  
        // For previous node of middle node 
        prev = one_node; 
  
        // Move one node each time
        two_node = two_node->next->next; 
  
        // Move two node each time
        one_node = one_node->next; 
    
  
    // Set middle node at head 
    prev->next = prev->next->next; 
    one_node->next = (*head); 
    (*head) = one_node; 
  
// To insert a node at the beginning of 
// linked list. 
void push(Node** head_ref, int new_data) 
    // Allocate node 
    Node* new_node = new Node(); 
  
    new_node->data = new_data; 
  
    // Link the old list off the new node 
    new_node->next = (*head_ref); 
  
    // Move the head to point to the new node 
    (*head_ref) = new_node; 
  
// A function to print a given linked list 
void printList(Node* ptr) 
    while (ptr != NULL)
    
        cout << ptr->data << " "
        ptr = ptr->next; 
    
    cout<<endl; 
  
// Driver code
int main() 
    // Create a list of 5 nodes 
    Node* head = NULL; 
    int i; 
    for (i = 5; i > 0; i--) 
        push(&head, i); 
  
    cout << " list before: "
    printList(head); 
  
    setMiddleHead(&head); 
  
    cout << " list After: "
    printList(head); 
  
    return 0; 
// This is code is contributed by rathbhupendra

Output:

list before: 1 2 3 4 5
list After : 3 1 2 4 5 

Please refer complete article on Make middle node head in a linked list for more details!


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