143. Reorder List

好题！！！虽然简单当时还是要想很多东西的！
3步

//Find the middle of the list
//Reverse the half after middle 1->2->3->4->5->6 to 1->2->3->6->5->4
//Start reorder one by one 1->2->3->6->5->4 to 1->6->2->5->3->4
/**
 * 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* p1 = head;
        ListNode* p2 = head;
        while(p2 -> next != NULL && p2 -> next -> next != NULL){
            p1 = p1 -> next;
            p2 = p2 -> next -> next;
        }

        ListNode* preM = p1;
        ListNode* preC = p1 -> next;
        while(preC -> next != NULL){
            ListNode* current = preC -> next;
            preC -> next = current -> next;
            current -> next = preM -> next;
            preM -> next = current;
        }

        p1 = head;
        p2 = preM -> next;
        while(p1 != preM){
            preM -> next = p2 -> next;
            p2 -> next = p1 -> next;
            p1 -> next = p2;
            p1 = p2 -> next;
            p2 = preM -> next;
        }
    }
};