bool isPalindrome(ListNode* head) {
int node_num = 0;
//O(n) compute list length
ListNode* tmp = head;
while (tmp != NULL) {
tmp = tmp->next;
++node_num;
}
if (node_num <= 1) {
return true;
}
//O(n) split into two lists
tmp = head;
ListNode* revert_head = NULL;
ListNode* forward_head = NULL;
ListNode* middle_node = NULL;
int current_index = 1;
int revert_head_index = node_num / 2;
int forward_head_index = (node_num + 1) / 2 + 1;
while (current_index < revert_head_index && tmp != NULL) {
tmp = tmp->next;
++current_index;
}
revert_head = tmp;
middle_node = tmp->next;
while (current_index < forward_head_index && tmp != NULL) {
tmp = tmp->next;
++current_index;
}
forward_head = tmp;
//O(n) revert the first half list
revertList(head, revert_head);
//O(n) compare two lists
ListNode* tmp_revert_node = revert_head;
ListNode* tmp_forward_node = forward_head;
bool is_palindrome = true;
while (tmp_revert_node != NULL) {
if (tmp_revert_node->val != tmp_forward_node->val) {
is_palindrome = false;
break;
}
tmp_revert_node = tmp_revert_node->next;
tmp_forward_node = tmp_forward_node->next;
}
//O(n) reset the list so that the original program can free the memory
revertList(revert_head, head);
revert_head->next = middle_node;
return is_palindrome;
}
int main()
{
PNode pLinkList = NULL;
for (int i = 0; i < 10; i++)
{
addNode(&pLinkList, i);
}
printf("print the list:\n");
printList(pLinkList);
revertList(&pLinkList);
printf("\n after being reverted:\n");
printList(pLinkList);
return 0;
}
int main(int argc, char const *argv[])
{
node *list = NULL;
for(int i = 1; i <= ST_ELEMENTOV; i++)
list = addItemEnd(list, i);
node *reverted = revertList(list);
print(list);
printf("------ Obrnjen: ------\n");
print(reverted);
free(reverted);
free(list);
return 0;
}
