[leetcode]Palindrome Linked List

细节实现题，在纸上画了一会才AC

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if(!head)   return true;
        ListNode* slow = head;
        ListNode* fast = head;
        ListNode* cnt = new ListNode(-1);
        ListNode* h = cnt;
        cnt->next = head;
        int len = 0;
        while(cnt->next){
            ++len;
            cnt = cnt->next;
        }
        if(len == 1)    return true;
        int half = (len + 1) >> 1;
        while(half){
            h = h->next;
            --half;
        }
        h = reverse(h);
        h = h->next;
        while(h){
            if(h->val != head->val) 
                return false;
            else{
                h = h->next;
                head = head->next;
            }
        }
        return true;
    }

    ListNode* reverse(ListNode* h){
        ListNode* pre = h->next;
        ListNode* cur = pre->next;
        ListNode* next = nullptr;
        while(cur){
            next = cur->next;
            cur->next = h->next;
            h->next = cur;
            pre->next = next;
            cur = next;
        }
        return h;
    }
};