题解 | 反转链表

#include<stdio.h>
#include<stdlib.h>
  
 
/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 * 
 * @param head ListNode类 
 * @return ListNode类
 */
/*方法一struct ListNode* ReverseList(struct ListNode* head ) {
    struct ListNode*first=NULL;
    struct ListNode*second=head;
    struct ListNode*third;
    while(second!=NULL){
        third=second->next;
        second->next=first;
        first=second;
        second=third;
    }
    return first;

}*///三指针迭代法
/*方法二struct ListNode* ReverseList(struct ListNode* head){
    if(head==NULL||head->next==NULL){
        return head;

    }
    struct ListNode* newhead=ReverseList(head->next);
        head->next->next=head;
        head->next=NULL;
    
return newhead;
}*/
/*方法三*/struct ListNode* ReverseList(struct ListNode* head){
    struct ListNode*newhead=NULL;
    struct ListNode*p;
    while(head!=0){
        p=head;
        head=head->next;
        p->next=newhead;
        newhead=p;
    }
    return newhead;
}