206. Reverse Linked List

Reverse a singly linked list.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)    return head;

        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *cur = head;
        ListNode *after = cur->next;

        while(after){
            //将after插入到dummy后面
            cur->next = after->next;
            after->next = dummy->next;
            dummy->next = after;

            //
            after = cur->next;
        }
        return dummy->next;
    }
};

遍历，头插

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* newHead = NULL;
        ListNode* temp =NULL;
        while(head != NULL){
            temp = head;
            head = head->next;

            //将节点插入到新链表的头部
            temp->next = newHead;
            newHead = temp;

        }
        return newHead;
    }
};

递归的方法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)   return head;

        ListNode *newHead = reverseList(head->next);
        //此时head->next已经成为上述链表的尾节点
        head->next->next = head;
        //这句也很重要啊
        head->next = NULL;

        return newHead;

    }
};