Algorithm: reverse the whole liked list

e.g., list=1->2->3->4->null

reverse it to: 4->3->2->1->Null;

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1. The procedure is shown below:
using three pointers: cur, pre and tmp. As following example shown, in the first step, put 2 in front of 1, then put 3 in front of 2, ........ until all are reversed. 

2. Note the case when only one node exist. In such a case, "temp = pre->next;" will be wrong! So this line of code should be within the while loop as the codes shown below. 

   

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//code

class Solution {

public:

       ListNode* reorderList(ListNode* head) {

              // reverse the link list

              //ListNode *dum = new ListNode(-1);

              //dum->next = head;

              ListNode* cur = head;

              ListNode* pre = head->next;

             

              cur->next = NULL;//the final node after reverse

              while (pre != NULL)

              {

                     ListNode* temp = pre->next;

                     pre->next = cur;

                     cur = pre;

                     pre = temp;

              }

              head = cur;

              return head;

       }

};