Sword refers to offer - merge two sorted linked lists

Q: Input two monotonically increasing linked lists, and output the linked list after combining the two linked lists. Of course, we need the combined linked list to satisfy the monotonically non-decreasing rule.

A: Select the linked list with the smaller element value of the first node as the basic linked list, and link the nodes in the other linked list to the basic linked list by constantly comparing the element values ​​of the two linked lists.

/*
struct ListNode {
	int val;
	struct ListNode *next;
	ListNode(int x) :
			val(x), next(NULL) {
	}
};*/
class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if(pHead1 == NULL && pHead2 == NULL)
        {
            return NULL;
        }
        if(pHead1 != NULL && pHead2 == NULL)
        {
            return pHead1;
        }
        if(pHead1 == NULL && pHead2 != NULL)
        {
            return pHead2;
        }
        
        // ListNode* tmp = new ListNode(0);
        ListNode* p1 = pHead1;
        ListNode* q1 = pHead1;
        ListNode* p2 = pHead2;
        ListNode* q2 = pHead2;
        if(pHead1->val <= pHead2->val)
        {
            while(p1 && p2)
            {
                q2 = p2;
                q1 = p1;
                p2 = p2->next;
                p1 = p1->next;
                q2->next = q1->next;
                q1->next = q2;
            }
            return pHead1;
        }
        else
        {
            while(p1 && p2)
            {
                q1 = p1;
                q2 = p2;
                p1 = p1->next;
                p2 = p2->next;
                q1->next = q2->next;
                q2->next = q1;
            }
            return pHead2;
        }
    }
};

Reference method 1:

#include<iostream>
#include <stdio.h>
#include <malloc.h>
#include <string.h>
using namespace std;

typedef struct ListNode{
    int value;
    struct ListNode* next;
}ListNode;

//print linked list
void OutPut(ListNode*head){
    if(head == NULL){
        printf("NULL\n");
    }
    else{
        ListNode *p;
        p = head->next;
        while(p != NULL){
            //if it is the first
            if(p->next == NULL){
                printf("%d\n",p->value);
            }
            else{
                printf("%d ",p->value);
            }
            p = p->next;
        }
    }
}
//create linked list
ListNode* CreateList(ListNode *head,int n){
    ListNode *newNode,*p;
    p = head;
    for(int i = 0;i < n;i++){
        newNode = (ListNode*)malloc(sizeof(ListNode));
        scanf("%d",&newNode->value);
        newNode->next = NULL;
        p->next = newNode;
        p = newNode;
    }
    return head;
}
ListNode* Merge(ListNode*head1,ListNode*head2){
    //head1, head2 with head node
    if(head1->next == NULL && head2->next == NULL){
        return NULL;
    }
    //Only the first string, no need to merge, output directly
    else if(head2->next == NULL){
        return head1;
    }
    //Only the second string, no need to merge, output directly
    else if(head1->next == NULL){
        return head2;
    }
    //merge
    else{
        ListNode *p1,*p2,*p3,*head;
        head = (ListNode*)malloc(sizeof(ListNode));
        head->next = NULL;
        p1 = head1->next;
        p2 = head2->next;
        p3 = head;
        while(p1 != NULL && p2 != NULL){
            if(p1->value < p2->value){
                p3->next = p1;
                p1 = p1->next;
            }
            else{
                p3->next = p2;
                p2 = p2->next;
            }
            p3 = p3->next;
        }
        //head1 has not been traversed
        while(p1 != NULL){
            p3->next = p1;
            p1 = p1->next;
            p3 = p3->next;
        }
        //head2 has not finished traversing
        while(p2 != NULL){
            p3->next = p2;
            p2 = p2->next;
            p3 = p3->next;
        }
        return head;
    }
}
int main() {
    int i,n,m;
    while(scanf("%d %d",&n,&m) != EOF){
        ListNode *head1,*head2;
        head1 = (ListNode*)malloc(sizeof(ListNode));
        head2 = (ListNode*)malloc(sizeof(ListNode));
        head1->next = NULL;
        head2->next = NULL;
        //create linked list
        if(n != 0){
            head1 = CreateList(head1,n);
        }
        if(m != 0){
            head2 = CreateList(head2,m);
        }
        // merge sort
        head1 = Merge(head1,head2);
        //print linked list
        if(head1 == NULL){
            printf("NULL\n");
        }
        else{
            OutPut(head1);
        }
    }//while
    return 0;
}

Reference method 2:

#include<iostream>
#include <stdio.h>
#include <malloc.h>
#include <string.h>
using namespace std;

typedef struct ListNode{
    int value;
    struct ListNode* next;
}ListNode;

//print linked list
void OutPut(ListNode*head){
    if(head == NULL){
        printf("NULL\n");
    }
    else{
        ListNode *p;
        p = head;
        while(p != NULL){
            //if it is the first
            if(p->next == NULL){
                printf("%d\n",p->value);
            }
            else{
                printf("%d ",p->value);
            }
            p = p->next;
        }
    }
}
//create linked list
ListNode* CreateList(ListNode *head,int n){
    ListNode *newNode,*p;
    p = head;
    for(int i = 0;i < n;i++){
        newNode = (ListNode*)malloc(sizeof(ListNode));
        scanf("%d",&newNode->value);
        newNode->next = NULL;
        p->next = newNode;
        p = newNode;
    }
    return head;
}
//recursive
ListNode* Merge(ListNode*head1,ListNode*head2){
    if(head1 == NULL && head2 == NULL){
        return NULL;
    }
    else if(head2 == NULL){
        return head1;
    }
    else if(head1 == NULL){
        return head2;
    }
    //merge
    ListNode *head = NULL;
    if(head1->value < head2->value){
        head = head1;
        head->next = Merge(head1->next,head2);
    }
    else{
        head = head2;
        head->next = Merge(head1,head2->next);
    }
    return head;
}
int main() {
    int i,n,m;
    while(scanf("%d %d",&n,&m) != EOF){
        ListNode *head1,*head2;
        head1 = (ListNode*)malloc(sizeof(ListNode));
        head2 = (ListNode*)malloc(sizeof(ListNode));
        head1->next = NULL;
        head2->next = NULL;
        //create linked list
        if(n != 0){
            head1 = CreateList(head1,n);
        }
        if(m != 0){
            head2 = CreateList(head2,m);
        }
        // merge sort
        head1 = Merge(head1->next,head2->next);
        //print linked list
        if(head1 == NULL){
            printf("NULL\n");
        }
        else{
            OutPut(head1);
        }
    }//while
    return 0;
}