给定一个单链表 L：L₀→L₁→…→L_n-1→L_n ， 将其重新排列后变为： L₀→L_n→L₁→L_n-1→L₂→L_n-2→…

你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。

示例 1:

给定链表 1->2->3->4, 重新排列为 1->4->2->3.

示例 2:

给定链表 1->2->3->4->5, 重新排列为 1->5->2->4->3.

参考答案

class Solution {
public:
    void reorderList(ListNode* head) {
        if (head == nullptr) {
            return;
        }
        ListNode* mid = middleNode(head);
        ListNode* l1 = head;
        ListNode* l2 = mid->next;
        mid->next = nullptr;
        l2 = reverseList(l2);
        mergeList(l1, l2);
    }

    ListNode* middleNode(ListNode* head) {
        ListNode* slow = head;
        ListNode* fast = head;
        while (fast->next != nullptr && fast->next->next != nullptr) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }

    ListNode* reverseList(ListNode* head) {
        ListNode* prev = nullptr;
        ListNode* curr = head;
        while (curr != nullptr) {
            ListNode* nextTemp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    void mergeList(ListNode* l1, ListNode* l2) {
        ListNode* l1_tmp;
        ListNode* l2_tmp;
        while (l1 != nullptr && l2 != nullptr) {
            l1_tmp = l1->next;
            l2_tmp = l2->next;

            l1->next = l2;
            l1 = l1_tmp;

            l2->next = l1;
            l2 = l2_tmp;
        }
    }
};