[LeetCode]Reorder List

/**
* 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) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if(head == NULL) return;
int listLen = getListLen(head);
ListNode* head1 = NULL;
ListNode* head2 = NULL;
divideList(head, listLen, head1, head2);
head2 = reverseList(head2);
mergeList(head1, head2);
}
private:
void divideList(ListNode* head, int listLen, ListNode*& head1, ListNode*& head2)
{
int firstHalfLen = (listLen+1)/2;
head1 = head;
ListNode* pFirstHalf = head;
for(int k = 1; k < firstHalfLen; ++k)
pFirstHalf = pFirstHalf->next;
head2 = pFirstHalf->next;
pFirstHalf->next = NULL;
}
ListNode* reverseList(ListNode* head)
{
if(head == NULL) return head;
ListNode dummy(-1);
ListNode* prev = &dummy;
prev->next = head;
ListNode* reverseHead = head;
ListNode* reverseTail = head;
ListNode* cur = head->next;

while(cur)
{
reverseTail->next = cur->next;
prev->next = cur;
cur->next = reverseHead;
reverseHead = cur;
cur = reverseTail->next;
}

return dummy.next;
}
void mergeList(ListNode* head1, ListNode* head2)
{
ListNode dummy(-1);
ListNode* prev = &dummy;
while(head1 || head2)
{
prev->next = head1;
head1 = head1->next;
prev = prev->next;

prev->next = head2;
if(head2)
{
head2 = head2->next;
prev = prev->next;
}

}
}
int getListLen(ListNode* head)
{
int len = 0;
while(head)
{
len++;
head = head->next;
}
return len;
}
};