143. 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) {
        if(head == NULL || head->next == NULL){
            return;
        }
        
        
        
        ListNode* fast = head;
        ListNode* slow = head;
        
        while(fast->next != NULL && fast->next->next != NULL){
            fast = fast->next->next;
            slow = slow->next;
        }
        fast = slow->next;
        slow->next = NULL;
        
        fast = ReverseList(fast);
        
        slow = head;
        
        while(slow){
            if(fast != NULL){
                ListNode* n = slow->next;
                slow->next = fast;
                ListNode* nn = fast->next;
                fast->next = n;
                fast = nn;
                slow = n;
            }else{
                break;
            }
        }
    }
    
    
    ListNode* ReverseList(ListNode* pHead){
        ListNode* pReversedHead = NULL;
        ListNode* pNode = pHead;
        ListNode* pPrev = NULL;
        
        while(pNode != NULL){
            ListNode* pNext = pNode->next;
            
            if(pNext == NULL){
                pReversedHead = pNode;
            }
            
            pNode->next = pPrev;
            pPrev = pNode;
            pNode = pNext;
        }
        return pReversedHead;
    }
};