LeetCode入门训练 回文链表

一种是直接将元素压入栈 然后依次退栈和链表相比

第二种是快慢指针，fast两格一跳，slow 一格一跳。在后半压入栈。与链表相比

第三种 直接在后半段取反 比较完后还原

/**
 * 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) {
        struct ListNode *p = NULL;
        struct ListNode *q;
        while (head) {
            q = head;
            head = head->next;
            q->next = p;
            p = q;
        }
        return p;
    }
    bool isPalindrome(ListNode* head) {
        
        if (head == NULL) return true;
        if (head->next == NULL) return true;
        if (head->next->next == NULL) {
            if (head->val == head->next->val) 
                return true;
            else return false;
        }
        ListNode *fast = head;
        ListNode *slow = head;
        while (fast->next != NULL && fast->next->next != NULL) {
            slow = slow->next;
            fast = fast->next->next;
        }
        if (fast->next !=NULL) {
            fast = fast->next;
        }
        ListNode *newhead = slow->next;
        slow->next = NULL;
        newhead = reverseList(newhead);
        ListNode *p = head;
        ListNode *q = newhead;
        while (p != NULL && q != NULL) {
            if (p->val != q->val)
                return false;
            p = p->next;
            q = q->next;
        }
        return true;
    }
};