Sort List

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

在O(n log n)时间对一个链表进行排序，首先想到的是几种时间复杂度为的O(n log n)排序算法，包括快排、归并排序、堆排序。

快速排序过程中，需要有两个指针指向首尾元素，并分别从首尾开始与基准元素进行比较，指针向后向前移动，比较适合双向链表；

其中归并排序比较适合单向链表的排序。

/** * 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 == NULL || head->next == NULL) {return head;} else {ListNode *slow = head;ListNode *fast = head;while(fast->next != NULL && fast->next->next != NULL) {fast = fast->next->next;slow = slow->next;}fast = slow;slow = slow->next;fast->next = NULL;fast = sortList(head); // 排左半段slow = sortList(slow);// 排右半段return merge(fast, slow); // 归并左右排序后的结果}}ListNode* merge(ListNode *head1, ListNode *head2) {if(head1 == NULL && head2 == NULL) return NULL;if(head1 == NULL) return head2;if(head2 == NULL) return head1;ListNode *ret, *p;if(head1->val < head2->val) {ret = head1;head1 = head1->next;} else {ret = head2;head2 = head2->next;}p = ret;while(head1 != NULL && head2 != NULL) {if(head1->val < head2->val) {p->next = head1;head1 = head1->next;} else {p->next = head2;head2 = head2->next;}p = p->next;}if(head1 != NULL) { // 如果head1还有元素p->next = head1;} else if (head2 != NULL) { // 如果head2还有元素p->next = head2;}return ret;}};