Leetcode | 148 Sort List

148 Sort List(排序链表）

题目：链接

一、归并排序：

class Solution {
public:
    ListNode *mergeSortList(ListNode *head) 
    {
        if(head == NULL || head->next == NULL)
            return head;
        else
        {   ////快慢指针找到中间节点
            ListNode *fast = head, *slow = 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 = mergeSortList(head); //前半段排序
            slow = mergeSortList(slow); //后半段排序
            return merge(fast,slow);
        }        
    }
    
    ListNode *merge(ListNode *head1, ListNode *head2)
    {
        if(head1 == NULL)
            return head2;
        if(head2 == NULL)
            return head1;
        ListNode *res, *p;
        if(head1->val < head2->val) 
            {
                res = head1; 
                head1 = head1->next;
            }
        else
            {
                res = head2; 
                head2 = head2->next;
            }
        p = res;    
        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)
            p->next = head1;
        else if(head2 != NULL)
            p->next = head2;
        return res;
    }
};