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// 234. Palindrome Linked List
#include "leetcode.h"
/*
* given the head of a signly linked list return true if it is a palindrome or
* false otherwise.
*/
struct ListNode {
int val;
struct ListNode *next;
};
struct ListNode *reverse_list(struct ListNode *head) {
if (!head || !head->next)
return head;
struct ListNode *tmp = reverse_list(head->next);
head->next->next = head;
head->next = NULL;
return tmp;
}
bool isPalindrome(struct ListNode *head) {
if (!head)
return 1;
struct ListNode *iter = head, *reversed_half_list;
int cnt = 0;
bool flag = 1;
while (iter) {
cnt++;
iter = iter->next;
}
iter = head;
for (int i = 0; i < cnt / 2; i++)
iter = iter->next;
if (!(cnt % 2))
reversed_half_list = reverse_list(iter);
else if (iter->next)
reversed_half_list = reverse_list(iter->next);
iter = head;
if (cnt == 1)
return 1;
if (!reversed_half_list)
return 0;
while (iter && reversed_half_list) {
if (iter->val != reversed_half_list->val) {
flag = 0;
break;
}
iter = iter->next;
reversed_half_list = reversed_half_list->next;
}
return flag;
}
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