[leetcode 148] Sort List

原创于 2015-01-06 12:40:44 发布 · 274 阅读

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#leetcode #Linked List #Sort

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本文介绍了一种使用归并排序思想实现的链表排序算法，该算法可以在O(nlogn)的时间复杂度内完成排序，并且仅使用常数级别的额外空间。通过递归地将链表分成两半，分别排序后再进行合并，最终实现整个链表的有序排列。

Sort a linked list in O(n log n) time using constant space complexity.

/**
 * 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 == nullptr || head->next == nullptr){
            return head;
        }
        auto *slow = head;
        auto *fast = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        auto head2 = slow->next;
        slow->next = NULL;
        auto left = sortList(head);
        auto right = sortList(head2);
        return merge(left,right);
    }
private:
    ListNode *merge(ListNode *l1, ListNode *l2) {
        ListNode dummy(-1);
        for (ListNode *p = &dummy; l1 || l2; p = p->next) {
            int val1 = l1?l1->val:INT_MAX;
            int val2 = l2?l2->val:INT_MAX;
            if (val1<val2) {
                p->next = l1;
                l1 = l1->next;
            } else {
                p->next = l2;
                l2 = l2->next;
            }
        }
        return dummy.next;
    }
};