234. Palindrome Linked List

Posted 2021-01-15 real1587

Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false

Example 2:

Input: 1->2->2->1
Output: true

Follow up:
Could you do it in O(n) time and O(1) space?
判断单链表是不是回文串，首先写了一个反转链表函数reverseList，为了省时间在isPalindrome函数里面用双指针一快一慢让慢的走到中间的结点，然后只需要反转后面的一半结点，再跟前面的一半比较即可知道是不是回文串。

 1 struct ListNode* reverseList(struct ListNode* head) {
 2     if(!head)   return NULL;
 3     struct ListNode *prev=NULL,*next;
 4     while(head){
 5         next=head->next;
 6         head->next=prev;
 7         prev=head;
 8         head=next;
 9     }
10     return prev;
11 }
12 bool isPalindrome(struct ListNode* head) {
13     if(!head || !head->next) return true;
14     
15     struct ListNode *fast=head,*slow=head;
16     //如果fast->next或者fast->next->next少一个判断都会导致慢指针走多一步导致结果错误
17     while(fast && fast->next && fast->next->next){
18         slow=slow->next;
19         fast=fast->next->next;
20     }
21     struct ListNode *mid=reverseList(slow->next);
22     slow->next=NULL;
23     while(mid){
24         if(mid->val!=head->val)
25             return false;
26         mid=mid->next;
27         head=head->next;
28     }
29     return true;
30 }