HACKERRANK merge-two-sorted-linked-lists Solution

| August 22, 2016

The correct, optimal and working solution for programming question merge-two-sorted-linked-lists on hackerrank

/*
 *  Author: Arpit Bhayani
 *  http://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;
}