leetcode 206. Reverse 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) {
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;
}
};

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;
}
};

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;
}
};