[LeetCode] Palindrome Linked List

Palindrome Linked List

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

Follow up:
Could you do it in O(n) time and O(1) space?

将链表拆成两半，后一半反转后与前一半一一对比，最后恢复链表。O(n) time and O(1) space！

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *reverseList(ListNode *head) {
        ListNode *pre = NULL, *cur = head, *tmp;
        while (cur != NULL) {
            tmp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
    bool isPalindrome(ListNode* head) {
        if (head == NULL || head->next == NULL) return true;
        ListNode *slow = head, *fast = head->next;
        while (fast != NULL) {
            fast = fast->next;
            if (fast != NULL) fast = fast->next;
            else break;
            slow = slow->next;
        }
        ListNode *head2 = slow->next;
        slow->next = NULL;
        head2 = reverseList(head2);
        ListNode *l1 = head, *l2 = head2;
        bool res = true;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val != l2->val) {
                res = false;
                break;
            } else {
                l1 = l1->next;
                l2 = l2->next;
            }
        }
        head2 = reverseList(head2);
        slow->next = head2;
        return res;
    }
};