Bubble Sort Doubly Link List - c++

Question

I want to sort doubly link list using bubble sort. It is having loop problem and also the swap is not taking place properly.
The problem is the greatest number should be at the end after one iteration of i. But it doesn't and stop in between. I tried various options but could not solve it. Please help me solve this problem
#include <iostream>
using namespace std;
class node{
public:
node *next;
node *prev;
int data;
node(int x, node *p, node *n)
{
next = n;
prev = p;
data = x;
}
};
class list{
private:
node *head;
node *tail;
public:
list()
{
head = NULL;
tail = NULL;
}
void addtoend(int x)
{
if(head == NULL)
{
head = new node (x,NULL,NULL);
tail = head;
}
else if(tail->prev == NULL)
{
tail = new node(x,head,NULL);
head->next = tail;
}
else
{
node *temp = tail;
tail = new node(x,tail,NULL);
temp->next = tail;
}
}
void bubblesort()
{
node *temp = head;
node *temp2 = temp;
for(int i=0;i<10;i++)
{
cout<<"I: "<<i<<endl;
temp = head;
while(1)
{
temp2 = temp;
if(temp->next->data < temp->data)
{
if(temp == head)
{
cout<<"HEAD"<<endl;
head = head->next;
node *after = head->next;
head->prev = NULL;
temp->prev = head;
head->next = temp;
temp->next = after;
after->prev = temp;
}
else if(temp->next == tail)
{
cout<<"TAIL"<<endl;
node *back = temp->prev;
temp->next = NULL;
tail->next = temp;
temp->prev = tail;
back->next = tail;
tail->prev = back;
tail = tail->next;
tail->next=NULL;
}
else
{
cout<<"ANY"<<endl;
node *back = temp->prev;
node *after = temp->next->next;
back->next=temp->next;
temp->prev = temp->next;
after->prev->next = temp;
after->prev->prev = back;
after->prev = temp;
temp->next = after;
}
}
display();
display0();
temp = temp2->next;
cout<<"HEAD: "<<head->data<<endl;
cout<<"Tail: "<<tail->data<<endl;
cout<<endl;
if(temp->next->next== NULL)
{
break;
}
}
}
}
void display()
{
node *temp = head;
while(temp != NULL)
{
cout<<temp->data<<" ";
temp = temp->next;
}
cout<<endl;
}
void display0()
{
node *temp = tail;
while(temp != NULL)
{
cout<<temp->data<<" ";
temp = temp->prev;
}
cout<<endl;
}
};
main()
{
list l1;
int x[10] = {17,15,8,12,10,5,4,1,7,2};
for(int i=0;i<10;i++)
{
l1.addtoend(x[i]);
}
l1.display();
l1.bubblesort();
l1.display();
}

Answer 1

How-to implement bubblesort algorithm for double-linked list
There are at least two ways to implement the bubblesort algorithm in c++ for a double-linked list. Swapping the content aka the data of two neighbored nodes as shown under I. and re-wiring the pointers as shown under II.:
The advantage of II. is that no temporary memory temp2 is required. Therefore this is the preferred solution as stated also by the OP.
I. Swapping data
Just replace the OP bubblesort function by this one:
void bubblesort()
{
bool is_sorted = false;
node *temp = head;
while (!is_sorted)
{
is_sorted = true;
temp = head;
while (temp->next != NULL)
{
if(temp->next->data < temp->data)
{
is_sorted = false;
int temp2 = temp->data;
temp->data = temp->next->data;
temp->next->data = temp2;
}
temp = temp->next;
}
}
display();
display0();
}
The algorithm uses a Boolean variable is_sorted to check if the sorting of the double-linked list is finished. Otherwise the bubbles are still rising. It swaps only the data values of the double linked list.
II. Re-wiring the pointers
This is the required solution for the OP and the preferred one:
void bubblesort()
{
bool is_sorted = false;
node *temp = head;
while (!is_sorted)
{
is_sorted = true;
temp = head;
while (temp->next != NULL)
{
if(temp->next->data < temp->data)
{
is_sorted = false;
if(temp == head)
{
//temp->prev->next = temp->next; // 1. (impossible)
temp->next->prev = NULL; // 2.
head = temp->next;
}
else
{
temp->prev->next = temp->next; // 1.
temp->next->prev = temp->prev; // 2.
}
temp->prev = temp->next; // 3.
if(temp->next == tail)
{
temp->next = NULL; // 4.
tail = temp;
//temp->next->prev = temp2; // 5. (impossible)
}
else
{
temp->next = temp->next->next; // 4.
temp->next->prev = temp; // 5.
}
temp->prev->next = temp; // 6.
temp = temp->prev;
}
temp = temp->next;
}
}
display();
display0();
}
Output
The output of the program with updated bubblesort functions is:
17 15 8 12 10 5 4 1 7 2
1 2 4 5 7 8 10 12 15 17
17 15 12 10 8 7 5 4 2 1
1 2 4 5 7 8 10 12 15 17
First line shows the input list without sorting. The second shows the sorting result in ascending order, while the third shows the descending order, which proves that the forward and reverse links of the list are still working. The last line is the sorted list output in the main function.
The original order of the list is lost, of course.
The full source code is also available on github.
Hope it helps.

Answer 2

This is the code that I submitted and it is working properly. I find my mistakes by creating a visual picture on paper. Also my instructor has instructed to swap nodes, in any question data swapping is not acceptable.
void bubblesort()
{
node *temp = head;
int len = length();
for(int i=0;i<(len - 1);i++)
{
cout<<i<<endl;
temp = head;
int c=0;
while(c<(len - i - 1))
{
cout<<"Temp: "<<temp->data<<" ";
cout<<"Temp: "<<temp->next->data<<" "<<endl;
if(temp->next->data < temp->data)
{
if(temp == head)
{
cout<<"HEAD"<<endl;
head = head->next;
node *after = head->next;
head->prev = NULL;
temp->prev = head;
head->next = temp;
temp->next = after;
after->prev = temp;
temp = head->next;
}
else if(temp->next == tail)
{
cout<<"TAIL"<<endl;
node *back = temp->prev;
temp->next = NULL;
tail->next = temp;
temp->prev = tail;
back->next = tail;
tail->prev = back;
tail = tail->next;
tail->next=NULL;
temp= tail;
}
else
{
cout<<"ANY"<<endl;
node *back = temp->prev;
node *after = temp->next->next;
back->next=temp->next;
temp->prev = temp->next;
after->prev->next = temp;
after->prev->prev = back;
after->prev = temp;
temp->next = after;
temp = after->prev;
}
}
else
temp = temp->next;
display();
display0();
cout<<"HEAD: "<<head->data<<endl;
cout<<"Tail: "<<tail->data<<endl;
cout<<endl;
c++;
}
}
}