LeetCode | Remove Nth Node From End of List（移除链表中倒数第n个结点）

题目：

Given a linked list, remove the nth node from the end of list and return its head.

For example,

Given linked list: 1->2->3->4->5, and n = 2.

After removing the second node from the end, the linked list becomes 1->2->3->5.

题目解析：

这道题目和【剑指offer】面试题15：链表中倒数第k个结点很相像。只是这里要删除结点，要考虑的情况更多一点。

虽然这道题目比较容易，但是有很多地方要注意：输入的是空指针怎么办？输入的是0怎么处理？（为0的时候，n-1不是-1，而是最大的正数，因为变量设为unsigned）当链表的个数正好是n个的时候怎么处理？那么就要删除第一个结点，头指针的值会改变。

基于以上的情况分析，才能写出可靠的代码

LinkList RemoveNth(LinkList *head,unsigned int n)
{
if(n == 0)
return *head;
if(*head == NULL){
printf("list is NULL\n");
return NULL;
}
LinkList p = *head;

for(int i = 0;i < n-1;i++){
p = p->next;
if(!p){
printf("lenth < n\n");
exit(1);
}
}
if(p->next == NULL){    //证明长度刚好是n
p = *head;
*head = (*head)->next;
free(p);
return *head;
}
p = p->next;
LinkList q = *head; //此时的q指向的是倒数第n个结点的父节点！
while(p->next){
p = p->next;
q = q->next;
}
p = q->next;    //p重新赋值为q的子节点
q->next = p->next;
free(p);
return *head;
}

完整的程序代码：

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

typedef struct LNode{
int data;
struct LNode *next;
}ListNode,*LinkList;

void CreateList(LinkList *head,int n)
{
for(int i = 0;i < n;++i){
LinkList p = (LinkList)malloc(sizeof(ListNode));
if(p == NULL){
printf("malloc error\n");
exit(1);
}
scanf("%d",&(p->data));
p->next = *head;
*head = p;
}
}

void DestoryList(LinkList *head)
{
if(*head == NULL)
return ;
LinkList p = *head;
LinkList q = p->next;

while(q){
free(p);
p = q;
q = p->next;
}
free(p);
*head = NULL;
}

void PrintList(LinkList head)
{
LinkList p = head;
while(p){
printf("%d ",p->data);
p = p->next;
}
printf("\n");
}

LinkList RemoveNth(LinkList *head,unsigned int n)
{
if(n == 0)
return *head;
if(*head == NULL){
printf("list is NULL\n");
return NULL;
}
LinkList p = *head;

for(int i = 0;i < n-1;i++){
p = p->next;
if(!p){
printf("lenth < n\n");
exit(1);
}
}
if(p->next == NULL){    //证明长度刚好是n
p = *head;
*head = (*head)->next;
free(p);
return *head;
}
p = p->next;
LinkList q = *head; //此时的q指向的是倒数第n个结点的父节点！
while(p->next){
p = p->next;
q = q->next;
}
p = q->next;    //p重新赋值为q的子节点
q->next = p->next;
free(p);
return *head;
}

int main()
{
LinkList head = NULL;
int n;

while(scanf("%d",&n) != EOF){
CreateList(&head,n);
PrintList(head);
printf("input the delete num:");
scanf("%d",&n);
RemoveNth(&head,n);
PrintList(head);
DestoryList(&head);
}

return 0;
}