Leetcode: Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,

reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,

Given 

{1,2,3,4}

, reorder it to 

{1,4,2,3}

.

/**
* 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==NULL || head->next==NULL)
return head;
ListNode* tmphead = reverseList(head->next);
head->next->next = head;
head->next = NULL;
return tmphead;
}
void reorderList(ListNode *head) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if(head==NULL || head->next==NULL)
return;
ListNode* pslow = head;
ListNode* pfast = head;
while(pfast!=NULL)
{
if(pfast != NULL)
pfast = pfast->next;
if(pfast != NULL)
pfast = pfast->next;
if(pfast==NULL)
break;
pslow = pslow->next;
}
ListNode* head2 = pslow->next;
pslow->next = NULL;
head2 = reverseList(head2);
ListNode* p1 = head;
ListNode* p2 = head2;
ListNode* tmp = NULL;
while(p1!=NULL && p2 != NULL){
tmp = p1->next;
p1->next = p2;
p1 = tmp;
tmp = p2->next;
p2->next = p1;
p2 = tmp;
}
}
};