回文链表

什么是回文？举两个栗子

上海自来水来自海上

黄山落叶松叶落山黄

如何判断链表是不是回文呢？

1. 找到链表的中间节点
2. 将链表后半部分反转
3. 比较链表前半部分和后半部分是否相同

struct ListNode {
    char value;
    ListNode *next;
};

ListNode* getMiddleNode(ListNode *head)
{
    if(head == NULL || head->next == NULL)
        return head;
    ListNode *fast = head;
    ListNode *slow = head;
    while (fast->next != NULL)
    {
        fast = fast->next;
        slow = slow->next;
        if(fast->next != NULL)
            fast = fast->next;
    }
    return slow;
}

ListNode* reverseListRecursive(ListNode *head)
{
    if(head == NULL || head->next == NULL)
        return head;
    ListNode *newHead = reverseListRecursive(head->next);
    head->next->next = head;
    head->next = NULL;
    return newHead;
}

bool isPalindrome(ListNode *head)
{
    if (head == NULL || head->next == NULL)
        return true;
    ListNode *middleNode = getMiddleNode(head);
    ListNode *rbegin = reverseListRecursive(middleNode);
    ListNode *lbegin = head;
    while (rbegin != NULL) {
        if (lbegin->value != rbegin->value) {
            return false;
        }
        lbegin = lbegin->next;
        rbegin = rbegin->next;
    }
    return true;
}

完美。