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:
void reorderList(ListNode *head) {

int len = 0;
ListNode *tmp = head;
while(tmp)
{
len++;
tmp = tmp->next;
}
if(head == NULL || len <= 2)
return;

ListNode *p1 = NULL;
ListNode *p2 = NULL;
int j = 0;
if(len%2 == 0)
j = len/2 - 1;
else
j = len/2;

tmp=head;
for(int i=0; i<j; i++)
tmp = tmp->next;

p1 = tmp;
p2 = tmp->next;
ListNode *reverse = NULL;
p1->next = NULL;
while(p2)
{
ListNode *p2next = p2->next;
ListNode *rnext = reverse;
reverse = p2;
p2->next = rnext;
p2 = p2next;
}
tmp = head;
while(reverse)
{
ListNode *next = reverse->next;
reverse->next = tmp->next;
tmp->next = reverse;
tmp = reverse->next;
reverse = next;
}
return;
}
};