迴文鏈表

什麼是迴文？舉兩個栗子

上海自來水來自海上

黃山落葉松葉落山黃

如何判斷鏈表是否是迴文呢？

1. 找到鏈表的中間節點
2. 將鏈表後半部分反轉
3. 比較鏈表前半部分和後半部分是否相同

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。