Merge Two Sorted Link Lists

Merge two sorted linked lists and return them as a sorted list. The list should be made by splicing together the nodes of the first two lists.

Example 1:

Input: l1=1->2->4->NULL, l2=1->3->4->5->NULL
Output: 1->1->2->3->4->4->5->NULL                                                                                                   

Approach

Java

public class MergeTwoSortedLinkList {
    public static void main(String[] args) {
        ListNode l1 = new ListNode(1);
        l1.next = new ListNode(2);
        l1.next.next = new ListNode(4);

        ListNode l2 = new ListNode(1);
        l2.next = new ListNode(3);
        l2.next.next = new ListNode(4);
        l2.next.next.next = new ListNode(5);
        ListNode head = mergeTwoLists(l1, l2);
        while(head!=null)
        {
            System.out.print(" "+head.val);
            head=head.next;
        }
    }

    public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode r = null;
        boolean flag = true;
        ListNode head = null;
        while (l1 != null || l2 != null) {
            if (l1 != null && l2 != null && l1.val < l2.val) {

                if (flag) {
                    head = l1;
                    r = head;
                    flag = false;
                } else {
                    r.next = l1;
                    r = l1;
                }
                l1 = l1.next;
            } else if (l1 != null && l2 != null && l1.val >= l2.val) {
                if (flag) {
                    head = l2;
                    r = head;
                    flag = false;
                } else {
                    r.next = l2;
                    r = l2;
                }

                l2 = l2.next;
            } else if (l1 != null && l2 == null) {

                if (flag) {
                    head = l1;
                    r = head;
                    flag = false;
                } else {
                    r.next = l1;
                    r = l1;
                }

                l1 = l1.next;
            } else {
                if (flag) {
                    head = l2;
                    r = head;
                    flag = false;
                } else {
                    r.next = l2;
                    r = l2;
                }

                l2 = l2.next;
            }

        }
        return head;

    }

}

class ListNode {

    int val;
    ListNode next;

    ListNode() {
    }

    ListNode(int val) {
        this.val = val;
    }

    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

// Time Complexity : O(n)
// Space Complexity : O(1)

C++

#include <bits/stdc++.h>
using namespace std;
//Structure of node
struct Node
{ 
   int data;
   Node *next;
   Node(int data)
    {
        this->data=data;
        this->next=NULL;
    }
};
//Function to merge two
//Sorted Linklist
Node* mergeTwoLinkLists(Node* l1, Node* l2) 
{
        Node *temp1=l1,*temp2=l2;
        if(temp1==NULL && temp2==NULL)
               return NULL;
        if(temp1==NULL)
               return temp2;
        if(temp2==NULL)
               return temp1;
        Node *head=NULL,*temp=NULL;
        int flag=0;
        while(temp1&&temp2)
        {
            if(temp1->data<temp2->data)
            {
                if(flag==0)
                {
                    head=temp1;
                    flag=1;
                    temp=head;
                }
            else
            {
                temp->next=temp1;
                temp=temp1;
            }
               temp1=temp1->next;
               
            }
          else
          {
              if(flag==0)
              {
                  head=temp2;
                  flag=1;
                  temp=head;
              }
                  else
            {
                temp->next=temp2;
                temp=temp2;
            }
              temp2=temp2->next;
          }
        }
        if(temp1)
              temp->next=temp1;
        else if(temp2)
              temp->next=temp2;
        return head;
}
int main()
{
  Node *l1=new Node(1);
  l1->next=new Node(2);
  l1->next->next=new Node(4);
  Node *l2=new Node(1);
  l2->next=new Node(3);
  l2->next->next=new Node(4);
  l2->next->next->next=new Node(5);
  Node *head=mergeTwoLinkLists(l1,l2);
  while(head!=NULL)
    {
      cout<<head->data<<" ";
      head=head->next;
    }
   return 0;
}
//Time Complexity: O(n)
//Space Complexity:O(1)