19.c

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

structListNode {
intval;
structListNode*next;
};

structListNode*removeNthFromEnd(structListNode*head, intn) {
if (n==0) returnhead;

structListNode*fast, *slow;
fast=slow=head;

while (n--) {
fast=fast->next;
 }

if (fast==NULL) { /* reach the tail, so we delete the head */
head=head->next;
returnhead;
 }

while (fast->next!=NULL) {
fast=fast->next;
slow=slow->next;
 }

/* slow pointer is at the previous node of the one to be deleted */
slow->next=slow->next->next;

returnhead;
}

intmain() {
structListNode*l1= (structListNode*)calloc(5, sizeof(structListNode));
structListNode*p=l1;
inti;
for (i=1; i <= 4; i++) {
p->val=i;
p->next=p+1;
p++;
 }
p->val=5;
p->next=NULL;

p=removeNthFromEnd(l1, 1); /* delete the tail */

while (p) {
printf("%d ", p->val);
p=p->next;
 }
printf("\n");


p=removeNthFromEnd(l1, 0); /* delete nothing */

while (p) {
printf("%d ", p->val);
p=p->next;
 }
printf("\n");

p=removeNthFromEnd(l1, 4); /* delete the head */

while (p) {
printf("%d ", p->val);
p=p->next;
 }
printf("\n");

return0;
}