21.c

#include<stdio.h>
#include<stdlib.h>

structListNode {
intval;
structListNode*next;
};

structListNode*mergeTwoLists(structListNode*l1, structListNode*l2) {
if (l1==NULL) returnl2;
if (l2==NULL) returnl1;

structListNode*p1=l1;
structListNode*p2=l2;

structListNode*ret=NULL;
structListNode**p=&ret;

while (1) {
if (p1&&p2) {
if (p1->val <= p2->val) {
*p=p1;
p1=p1->next;
 }
else {
*p=p2;
p2=p2->next;
 }
 }
elseif (p1&&p2==NULL) {
*p=p1;
p1=p1->next;
 }
elseif (p2&&p1==NULL) {
*p=p2;
p2=p2->next;
 }
elsebreak;

p=&((*p)->next);
 }

returnret;
}

intmain() {

structListNode*headA
= (structListNode*)calloc(5, sizeof(structListNode));
structListNode*headB
= (structListNode*)calloc(3, sizeof(structListNode));

structListNode**p=&headA;
inti;
for (i=1; i <= 5; i++) {
 (*p)->val=i;
 (*p)->next=*p+1;
p=&(*p)->next;
 }
*p=NULL;

p=&headB;
for (i=3; i <= 7; i+=2) {
 (*p)->val=i;
 (*p)->next=*p+1;
p=&(*p)->next;
 }
*p=NULL;

printf("List A: ");
structListNode*q=headA;
while (q!=NULL) {
printf("%d->", q->val);
q=q->next;
 }
printf("N\n");

printf("List B: ");
q=headB;
while (q) {
printf("%d->", q->val);
q=q->next;
 }
printf("N\n");

structListNode*ret=mergeTwoLists(headA, headB);

printf("Merged: ");
q=ret;
while (q) {
printf("%d->", q->val);
q=q->next;
 }
printf("N\n");

return0;
}