143.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
    ListNode* reverse(ListNode* head) {
        ListNode *pre = nullptr, *next = nullptr;
        for (auto cur = head; cur != nullptr;) {
            next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }

  public:
    void reorderList(ListNode* head) {
        if (!head)
            return;
        head->next = reverse(head->next);
        reorderList(head->next);
    }
};