leetcode206. Reverse Linked List

Reverse Linked List

1. 迭代法：

/**
 1. Definition for singly-linked list.
 2. struct ListNode {
 3.     int val;
 4.     struct ListNode *next;
 5. };
 */
#define null 0
struct ListNode* reverseList(struct ListNode* head) {
    int i;
    struct ListNode *p,*q,*r;
    if(head==null)
        return null;
    r=head->next;
    p=head;
    q=null;
    while(r!=null){
        p->next=q;
        q=p;
        p=r;
        r=r->next;
    }
    p->next=q;
    return p;
}

1. 递归法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
#define null 0
struct ListNode* reverseList(struct ListNode* head) {
    struct ListNode *p;
    struct ListNode *q;
    if(!head || !(head->next))
        return head;
    p=reverseList(head->next);
    head->next->next=head;
    head->next=null;
    return p;
}