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 may not modify the values in the list's nodes, only nodes itself may be changed.

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3.

Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

重新排列链表

1->2->3->4->5 to 1->5->2->4->3.

C++：

/**
 * 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 ;
        }
        
        //1->2->3->4->5->6 to 1->2->3->6->5->4
        ListNode* head2 = p1->next ;
        p1->next = NULL ;
        
        p2 = head2->next ;
        head2->next = NULL;
        while(p2 != NULL){
            p1 = p2->next ;
            p2->next = head2 ;
            head2 = p2 ;
            p2 = p1 ;
        }

        // 1->2->3->6->5->4 to 1->6->2->5->3->4
        p1 = head ;
        p2 = head2 ;
        while(p1 != NULL){
            ListNode* t = p1->next ;
            p1->next = p2 ;
            p1 = p2 ;
            p2 = t ;
        }
    }
};