9. Sort List && Insertion Sort List （链表排序总结）

Sort List

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

                  Have you been asked this question in an interview?                   Yes               说明：归并排序： 时间 O(nlogn)，空间 O(1). 每次将链表一分为二， 然后再合并。快排（用两个指针）

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
ListNode* getMid(ListNode *head) {
    ListNode *first, *second, *preFirst;
    preFirst = first = second = head;
    while(second != NULL) {
        preFirst = first;
        first = first->next;
        if(second->next == NULL) break;
        second = second->next->next;
    }
    preFirst->next = NULL;
    return first;
}
 
void Merge(ListNode *head1, ListNode *head2) {
    if(head1->val > head2->val) {
		int tem = head1->val;
		head1->val = head2->val;
		head2->val = tem;
	}
    ListNode *p = head1;
    while(p->next != NULL && head2 != NULL) {
        if(p->next->val >= head2->val) {
            ListNode *q = p->next;
            p->next = head2;
            head2 = head2->next;
            p->next->next = q;
            p = p->next;
        } else {
            p = p->next;
        }
    }
    if(head2 != NULL)
        p->next = head2;
}
 
void MergeSort(ListNode *head) {
    if(head == NULL || head->next == NULL) return;
    ListNode *mid = getMid(head);
	MergeSort(head);
	MergeSort(mid);
    Merge(head, mid);
}
 
class Solution {
public:
    ListNode *sortList(ListNode *head) {
       MergeSort(head);
       return head;
    }
};

Insertion Sort List

Sort a linked list using insertion sort.

说明： 与顺序表不同的时，每次找插入位置时从头开始走。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *insertionSortList(ListNode *head) {
        ListNode *p, *q;
        for(q = head; q != NULL; q = q->next) {
            for(p = head; p != q; p = p->next) {
                if(p->val >= q->val) {
                    int tem = p->val;
                    p->val = q->val;
                    q->val = tem;
                }
            }
        }
        return head;
    }
};