148. Sort List

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题目

https://leetcode.com/problems/sort-list/description/

-w956

想法

归并排序

链表操作比较烦

答案

自己写的一个归并排序，好多地方应该可以优化，不过AC了，就不说什么了：

不过用了递归，这个空间复杂度，恐怕不是O(1)

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class Solution {
private:
    ListNode *mergeSort(ListNode *head, int len) {
        if (len == 1) {
            head->next = NULL;
            return head;
        }
        if (len == 2) {
            if (head->val <= head->next->val) {
                head->next->next = NULL;
                return head;
            } else {
                ListNode *newHead = head->next;
                newHead->next = head;
                newHead->next->next = NULL;
                return newHead;
            }
        }
        int mid = len / 2;
        ListNode *p = head;
        for (int i = 0; i < mid; i++) {
            p = p->next;
        }
        ListNode *headL = mergeSort(head, mid);
        ListNode *headR = mergeSort(p, len - mid);
        ListNode dummy(0);
        p = &dummy;
        while (headL && headR) {
            if (headL->val <= headR->val) {
                p->next = headL;
                headL = headL->next;
            } else {
                p->next = headR;
                headR = headR->next;
            }
            p = p->next;
        }
        while (headL) {
            p->next = headL;
            headL = headL->next;
            p = p->next;
        }
        while (headR) {
            p->next = headR;
            headR = headR->next;
            p = p->next;
        }
        return dummy.next;
    }
public:
    ListNode *sortList(ListNode *head) {
        if (!head)
            return NULL;
        int len = 0;
        ListNode *p = head;
        while (p) {
            len++;
            p = p->next;
        }
        ListNode *ret = mergeSort(head, len);
        return ret;
    }
};