Reverse the adjacent nodes in a linked list

Reverse the adjacent nodes of a linked list as described below in the sample input and output:
Input 1: 1->2->3->4->5
Output1: 2->1->4->3->5
Input2: 1->2->3->4->5->6
Output2: 2->1->4->3->6->5

#include<iostream>
#include<stdlib.h>
using namespace std;

struct node
{
    int data;
    struct node* next;
};

struct node* createlist(int length)
{
    struct node *root=NULL,*end=NULL;
    for(int i=1;i<=length;i++)
    {
        struct node* new_node = (struct node*)malloc(sizeof(struct node));
        new_node->data=i;
        new_node->next=NULL;
        if(i==1)
        {
            root=new_node;
            end=root;
        }
        else
        {
            end->next=new_node;
            end=new_node;
        }
    }
    return root;
}
struct node* reverseadjacent(struct node* root)
{
    int length=0;
    struct node *temp=root;
    while(temp!=NULL)
    {
        length++;
        temp=temp->next;
    }
    if(length<2)
    {
        return root;
    }
    struct node *first=root,*sec=first->next,*third=sec->next;
    root=first->next;
    if(length==2)
    {
        sec->next=first;
        first->next=NULL;
    }
    else if(length%2==0)
    {
        while(third!=NULL)
        {
            first->next=third->next;
            sec->next=first;
            first=third;
            sec=third->next;
            third=sec->next;
        }
        sec->next=first;
        first->next=NULL;
    }
    else
    {
        while(third->next!=NULL)
        {
            first->next=third->next;
            sec->next=first;
            first=third;
            sec=third->next;
            third=sec->next;
        }
        sec->next=first;
        first->next=third;
    }
    return root;
}
void printlist(struct node* root)
{
    struct node* temp=root;
    while(temp!=NULL)
    {
        cout<<temp->data<<"->";
        temp=temp->next;
    }
    cout<<endl;
}
int main()
{
    struct node* root=createlist(5);
    cout<<"The given list\n";
    printlist(root);
    struct node* new_root=reverseadjacent(root);
    cout<<"The new list\n";
    printlist(new_root);
}