234. Palindrome Linked List

典中典之回文，不过是Linked List的回文~

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode *head) {
        if (head==nullptr || head->next==nullptr) return true;
        ListNode *fast = head;
        ListNode *slow = head;
        while(fast->next && fast->next->next){
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode *prev = nullptr;
        ListNode *succ;
        ListNode *cur = slow;
        while(cur){
            succ = cur->next;
            cur->next = prev;
            prev = cur;
            cur  = succ;
        }
        while(prev->next){
            if (prev->val!=head->val)
                return false;
            prev = prev->next;
            head = head->next;
        }
        return true;
    }
};

快慢指针法：快指针到头时慢指针刚好到链表的中间；接着用慢指针遍历后半段并将其倒置；最后比较对称位置的值是否相等。