206. Reverse Linked List

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?
反转链表。有两种方法可以解决:

1.迭代

struct ListNode* reverseList(struct ListNode* head) {
    if(!head)   return NULL;
    struct ListNode *prev=NULL,*next;
    while(head){
        next=head->next;
        head->next=prev;
        prev=head;
        head=next;
    }
    return prev;
}

2.递归

//递归方式
Node * reverseList(List head)
{
    //如果链表为空或者链表中只有一个元素
    if(head == NULL || head->next == NULL)
    {
        return head;
    }
    else
    {
        //先反转后面的链表，走到链表的末端结点
        Node *newhead = reverseList(head->next);
        //再将当前节点设置为后面节点的后续节点
        head->next->next = head;
        //原本的head指向关系给设置成NULL，不然head会指向后面结点报错 
        head->next = NULL;
        
        return newhead;
    }
}