# MERGE-TWO-SORTED-LINKED-LISTS hackerrank Solution - Correct, Optimal and Working

``````/*
*  Author: Arpit Bhayani
*  https://arpitbhayani.me
*/
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <climits>
#include <deque>
#include <iostream>
#include <list>
#include <limits>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <vector>

#define ll long long

#define MIN(a, b) a < b ? a : b
#define MAX(a, b) a > b ? a : b

using namespace std;

int readline(char *str) {
int i = 0;
char ch;
while((ch = getchar()) != '\n') {
str[i++] = ch;
}
str[i] = '\0';
return i;
}

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

void Print(struct Node *head) {
struct Node *p = head;
while(p) {
printf("%d\n", p->data);
p = p->next;
}
}

struct Node * new_node(int v) {
struct Node * p = (struct Node *) calloc(1, sizeof(struct Node));
p->data = v;
return p;
}

struct Node * MergeLists(struct Node * headA, struct Node * headB) {
struct Node * head = NULL;

struct Node * p = headA;
struct Node * q = headB;

if(p == NULL && q == NULL) {
return NULL;
}

if(p == NULL && q != NULL) {
head = q;
q = q->next;
}

if(p != NULL && q == NULL) {
head = p;
p = p->next;
}

if(p && q && p->data <= q->data) {
head = p;
p = p->next;
}
else if(p && q && q->data <= q->data) {
head = q;
q = q->next;
}

struct Node * r = head;

while(p && q) {
if(p->data <= q->data) {
r->next = p;
p = p->next;
}
else {
r->next = q;
q = q->next;
}
r = r->next;
}

while(p) {
r->next = p;
p = p->next;
r = r->next;
}

while(q) {
r->next = q;
q = q->next;
r = r->next;
}

r->next = NULL;
return head;
}

int main(int argc, char *argv[]) {
struct Node * headA = new_node(1);
headA->next = new_node(2);
headA->next->next = new_node(4);

struct Node * headB = new_node(1);
headB->next = new_node(3);

Print(MergeLists(headA, NULL));
return 0;
}
``````