Merge sort for single linked lists

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mahmoud2506

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2022-05-05

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Write a C++ program to sort two single linked lists using merge sort technique.

Solution: Merge sort is an efficient technique for sorting linked lists using recursion.

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mahmoud2506

2022-05-05

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C++ implementation for Merge sort for single linked lists

#include<bits/stdc++.h>
using namespace std;

class node{
    public:
	int data; // data field
	struct node *next;
};

void display(class node* head){
	node* current=head; // current node set to head
	while(current!=NULL){ //traverse until current node isn't NULL
		printf("%d ",current->data);
		current=current->next; // go to next node
	}
}

node* creatnode(int d){
	node* temp=(node*)malloc(sizeof(node));
	temp->data=d;
	temp->next=NULL;
	return temp;
}
node* mergeList(node* split1,node* split2){
    //merging two sorted list
    node* newhead=NULL;
    //base cases
    if(split1==NULL)
    return split2;
    if(split2==NULL)
    return split1;
    //recursively merge
    if(split1->data<=split2->data){
        newhead=split1;
        newhead->next=mergeList(split1->next,split2);
    }
    else{
        newhead=split2;
        newhead->next=mergeList(split1,split2->next);
    }
        
    return newhead;
    
}

void splitList(node* head,node** split1,node** split2){
    //similar to flyod's tortoise algorithm
    node* slow=head;
    node* fast=head->next;
    
    while(fast!=NULL){
        fast=fast->next;
        if(fast!=NULL){
            slow=slow->next;
            fast=fast->next;
        }
    }
    
    *split1=head;
    *split2=slow->next;
    //spliting
    slow->next=NULL;
}

void mergeSort(node** refToHead){
    node* head=*refToHead;
    node* split1,*split2;
    //base case
    if(head==NULL || head->next==NULL){
        return;
    }
    //split the list in two halves
    splitList(head,&split1,&split2);
    //recursively sort the two halves
    mergeSort(&split1);
    mergeSort(&split2);
    //merge two sorted list
    *refToHead=mergeList(split1,split2);
    
    return;
    
}

int main(){
	printf("creating the linked list by inserting new nodes at the end\n");
	printf("enter 0 to stop building the list, else enter any integer\n");
	int k,count=1,x;
	node* curr,*temp;
	scanf("%d",&k);
	node* head=creatnode(k); //buliding list, first node
	scanf("%d",&k);
	temp=head;

	///////////////////inserting at the end//////////////////////
	while(k){
		curr=creatnode(k);
		temp->next=curr;//appending each node
		temp=temp->next;
		scanf("%d",&k);
	}
	cout<<"before merge sort...\n";
	display(head); // displaying the list
	cout<<"\nafter merge sort...\n";
	mergeSort(&head);
	display(head);
	return 0;
}

Output

creating the linked list by inserting new nodes at the end
enter 0 to stop building the list, else enter any integer
6 5 4 7 1 0
before merge sort...
6 5 4 7 1
after merge sort...
1 4 5 6 7

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