LeetCode 328. Odd Even Linked List

Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we are talking about the node number and not the value in the nodes.

You should try to do it in place. The program should run in O(1) space complexity and O(nodes) time complexity.

Example:

Given

1->2->3->4->5->NULL

,

return

1->3->5->2->4->NULL

.

Note:

The relative order inside both the even and odd groups should remain as it was in the input.

The first node is considered odd, the second node even and so on ...

一、算法分析

设置两个尾指针，分别指向odd链表和even链表尾节点；从第三个开始遍历链表，根据flag判断是偶数节点还是奇数节点，然后分别添加到偶链表和奇链表尾部。

二、C语言实现

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     struct ListNode *next;
* };
*/
struct ListNode* oddEvenList(struct ListNode* head) {
struct ListNode *p,*rear1,*rear2,*temp;
int flag=-1;
if(head==NULL || head->next==NULL || head->next->next==NULL){
return head;
}
rear1=head;
rear2=head->next;
p=rear2->next;
while(p){
temp=p->next;
if(flag==-1){//表示是odd，加入到第一个链表尾部
p->next=rear1->next;
rear1->next=p;
rear1=p;
p=temp;
flag*=-1;
}else{
p->next=rear2->next;
rear2->next=p;
rear2=p;
p=temp;
flag*=-1;
}
}
rear2->next=NULL;
return head;

}