86. Partition List

Posted 2020-12-26 zhuangbijingdeboke

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution 
10 {
11 public:
12     ListNode* partition(ListNode* head, int x) 
13     {
14         ListNode *small=head,*big=head,*pre,*rpre;
15         ListNode *res=new ListNode(0);
16         res->next=head;
17         pre=res;
18         while(big!=NULL&&big->val<x)
19         {
20             big=big->next;
21             pre=pre->next;
22         }
23         rpre=pre,small=big;
24         while(small!=NULL)
25         {
26             if(small->val<x)
27             {
28                 rpre->next=small->next;
29                 pre->next=small;
30                 small->next=big;
31                 pre=pre->next;
32                 small=rpre->next;
33             }
34             else
35             {
36                 small=small->next;
37                 rpre=rpre->next;
38             }
39         }
40         return res->next;
41     }
42 };

核心思路是把所有比给定值小的节点摘到第一个大于等于给定值的节点左边

也可以直接新建两个链表，把小的和大的分开放，然后拼在一起

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution 
10 {
11 public:
12     ListNode* partition(ListNode* head, int x) 
13     {
14         ListNode *psmall=new ListNode(0);
15         ListNode *pbig=new ListNode(0);
16         ListNode *res=psmall,*k=pbig;
17         while(head!=NULL)
18         {
19             if(head->val<x)
20                 psmall=psmall->next=head;
21             else
22                 pbig=pbig->next=head;
23             head=head->next;
24         }
25         pbig->next=NULL;
26         psmall->next=k->next;
27         return res->next;
28     }
29 };