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Merge Sort for Doubly Linked List using C++.
Problem statement
Given a doubly linked list. Sort it using merge sort algorithm
List: 10->20->8-17->5->13->4 Sorted list: 4->5->8->10->13->17->20
Algorithm
1. If head is NULL or list contains only one element then return list 2. Create two lists by dividing original list into 2 parts 3. Sort first and second part of list 4. Merge both sorted list
Example
#include <iostream>
#include <new>
#define SIZE(arr) (sizeof(arr) / sizeof(arr[0]))
using namespace std;
struct node {
int data;
struct node *next;
struct node *prev;
};
node *createList(int *arr, int n){
node *head, *p, *q;
p = head = new node;
head->data = arr[0];
head->prev = NULL;
head->next = NULL;
for (int i = 1; i < n; ++i) {
q = new node;
q->data = arr[i];
q->prev = p;
q->next = NULL;
p->next = q;
p = q;
}
return head;
}
void displayList(node *head){
while (head != NULL) {
cout << head->data << " ";
head = head->next;
}
cout << endl;
}
node *mergeSortedLists(node *head1, node *head2){
node *result = NULL;
if (head1 == NULL) {
return head2;
}
if (head2 == NULL) {
return head1;
}
if (head1->data < head2->data) {
head1->next = mergeSortedLists(head1->next, head2);
head1->next->prev = head1;
head1->prev = NULL;
return head1;
} else {
head2->next = mergeSortedLists(head1, head2->next);
head2->next->prev = head2;
head2->prev = NULL;
return head2;
}
}
void splitList(node *src, node **fRef, node **bRef){
node *fast;
node *slow;
slow = src;
fast = src->next;
while (fast != NULL) {
fast = fast->next;
if (fast != NULL) {
slow = slow->next;
fast = fast->next;
}
}
*fRef = src;
*bRef = slow->next;
slow->next = NULL;
}
void mergeSort(node **head){
node *p = *head;
node *a = NULL;
node *b = NULL;
if (p == NULL || p->next == NULL) {
return;
}
splitList(p, &a, &b);
mergeSort(&a);
mergeSort(&b);
*head = mergeSortedLists(a, b);
}
int main(){
int arr[] = {10, 20, 8, 17, 5, 13, 4};
node *head;
head = createList(arr, SIZE(arr));
cout << "Unsorted list: " << endl;
displayList(head);
mergeSort(&head);
cout << "Final sorted list: " << endl;
displayList(head);
return 0;
}Output
When you compile and execute the above program. It generates the following output −
Unsorted list: 10 20 8 17 5 13 4 Final sorted list: 4 5 8 10 13 17 20
Updated on: 31-Oct-2019
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