【Leetcode】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}.

快慢指针找到链表中点，将链表分为两段，翻转后半段，再合并两个子链表。

/**
 * 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 == nullptr || head->next == nullptr || head->next->next == nullptr) {
            return;
        }
        ListNode *fast = head, *slow = head;
        while (fast->next && fast->next->next) {
            fast = fast->next->next;
            slow = slow->next;
        }
        fast = slow->next;
        slow->next = nullptr;
        fast = reverseList(fast);
        head = mergeList(head, fast);
    }
private:
    ListNode * reverseList(ListNode *head) {
        ListNode dummy(-1);
        ListNode *p = &dummy;
        while (head) {
            ListNode *temp = head->next;
            head->next = p->next;
            p->next = head;
            head = temp;
        }
        return dummy.next;
    }
    
    ListNode *mergeList(ListNode *l1, ListNode *l2) {
        ListNode dummy(-1);
        ListNode *p = &dummy;
        while (l1) {
            p->next = l1;
            l1 = l1->next;
            p = p->next;
            if (l2) {
                p->next = l2;
                l2 = l2->next;
                p = p->next;
            }
        }
        return dummy.next;
    }
};