13. Reorder List

Reorder List

           Given a singly linked list L: L₀→L₁→…→L_n-1→L_n, reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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}.

说明：分三步，1。从中间分开，成两部分。2，后半部分链表逆置。3.插入合并。

/**
 * 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) return; 
        ListNode *p1, *p2, *p3, *head2;
        p1 = p2 = head;
        while(p2->next && p2->next->next) {
            p1 = p1->next;
            p2 = p2->next->next;
        }
        head2 = p1->next; p1 = p1->next = NULL;
        p2 = head2;
        while(p2) {
            p3 = p2->next;
            p2->next = p1;
            p1 = p2;
            p2 = p3;
        }
        head2 = p1; p1 = head;
        while(head2 && p1) {
            p2 = head2->next;
            head2->next = p1->next;
            p1->next = head2;
            head2 = p2;
            p1 = p1->next->next;
        }
        
        
    }
};