Leetcode|找单链表中点+链表反转+链表合并|143. 重排链表

class Solution {
public:
    void reorderList(ListNode* head) {
        if (!head) return;
        // 1.寻找链表中点(快慢指针)
        auto premid = findmid(head);
        // 2.链表反转(pre/cur/next)
        reverse(premid);
        auto l1 = head;
        auto l2 = premid->next;
        premid->next = nullptr;
        // 3.链表合并(先保存next然后穿针引线)
        merge(l1, l2);
    }
	// 合并链表
    void merge(ListNode* l1, ListNode* l2) {
        while (l1 && l2) {
            // 先保存两个链表的next
            auto l1next = l1->next;
            auto l2next = l2->next;
            // 穿针引线
            l1->next = l2;
            l2->next = l1next;
            // 两指针同时前移
            l1 = l1next;
            l2 = l2next;
        }
    }
	// 反转链表
    void reverse(ListNode* prehead) {
        auto pre = prehead;
        auto cur = prehead->next;
        while (cur && cur->next) {
            auto next = cur->next;
            cur->next = next->next;
            next->next = pre->next;
            pre->next = next;
        }
    }
	// 找链表中点
    ListNode* findmid(ListNode* head) {
        auto fast = head, slow = head;
        while (fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
};