【归并排序+递归】LeetCode 148. Sort List

LeetCode 148. Sort List

Solution1：我的答案
很多递归

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if (!head) return NULL;
        if (!head->next) return head;
        ListNode *slow = head, *fast = head;
        while (fast->next && fast->next->next) {//快慢指针找链表中点
            slow = slow->next;
            fast = fast->next->next;
        }
        fast = slow->next;//fast重新定义为第二段链表的起点
        slow->next = NULL;//两条链表断开
        ListNode *l1 = sortList(head);
        ListNode *l2 = sortList(fast);
        return mergetwolists(l1, l2);
    }

    ListNode* mergetwolists (ListNode* l1, ListNode* l2) {//合并两个有序链表的代码重用度很高啊
        if (!l1) return l2;
        if (!l2) return l1;
        ListNode *dummy = new ListNode(-1), *cur = dummy;
        while (l1 && l2) {
            if (l1->val <= l2->val) {
                cur->next = l1;
                l1 = l1->next;
            } else {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        if (l1) cur->next = l1;
        if (l2) cur->next = l2;
        return dummy->next;
    }
};