leetcode 206. Reverse Linked List

法一：

/**
 * 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* rev_head = NULL;
        ListNode* tmp1 = NULL;
        ListNode* tmp2 = head;

        while(tmp2 != NULL)
        {
            // if(tmp2)
            //    cout << (tmp2->val) << endl;
            if(rev_head == NULL)
            {
                rev_head = tmp2;
                tmp2 = tmp2->next;
                rev_head->next = NULL;
            }
            else
            {
                tmp1 = rev_head;
                rev_head = tmp2;
                tmp2 = tmp2->next;
                rev_head->next = tmp1;   
            }
        }

        return rev_head;
    }
};

法二：

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* new_head = new ListNode(0);
        new_head -> next = head;
        ListNode* pre = new_head;
        ListNode* cur = head; 
        while (cur && cur -> next) {
            ListNode* temp = pre -> next;
            pre -> next = cur -> next;
            cur -> next = cur -> next -> next; 
            pre -> next -> next = temp;
        }
        return new_head -> next;
    }
};

法三：

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* pre = NULL;
        while (head) {
            ListNode* next = head -> next;
            head -> next = pre;
            pre = head;
            head = next;
        } 
        return pre;
    }
};