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

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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;}

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