【Reorder List】cpp

/**
 * 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) return;
        ListNode dummy(-1);
        dummy.next = head;
        ListNode *p1 = &dummy, *p2 = &dummy;
        // get the mid Node
        for (; p2 && p2->next; p1 = p1->next, p2 = p2->next->next);
        // reverse the second half list
        for ( ListNode *prev = p1, *curr = p1->next; curr && curr->next; ){
            ListNode *tmp = curr->next;
            curr->next = curr->next->next;
            tmp->next = prev->next;
            prev->next = tmp;
        }
        // cut the list from mid and merge two list
        for ( p2 = p1->next, p1->next = NULL,p1 = head; p2; ){
            ListNode *tmp = p1->next;
            p1->next = p2;
            p2 = p2->next;
            p1->next->next = tmp;
            p1 = tmp;
        }
    }
};