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 || !head->next || !head->next->next) return;
        
        //find the medium node
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast && fast->next){
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode* secondPre = slow;
        ListNode* secondStart = slow->next;
        secondPre->next = NULL; //break the list into two parts
        
        //reverse the second part
        ListNode* secondMove = secondStart->next;
        secondStart->next = NULL; //generalize the second list
        while(secondMove){
            ListNode* secondAfter = secondMove->next;
            secondMove->next = secondStart;
            secondStart = secondMove;
            secondMove = secondAfter;
        }
        //now secondStart is the begining of the second list
        
        //combine two lists
        ListNode* firstStart = head;
        while(firstStart && secondStart){
            ListNode* firstTemp = firstStart;
            firstStart = firstStart->next;
            ListNode* secondTemp = secondStart;
            secondStart = secondStart->next;
            firstTemp->next = secondTemp;
            secondTemp->next = firstStart;
            //firstStart->next = secondStart;
        }
    }
};