palindromeLinkedList.cpp

// Source: https://leetcode.com/problems/palindrome-linked-list/
// Author: Miao Zhang
// Date:   2021-01-28

/**
 * 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) {
        ListNode* slow = head;
        ListNode* fast = head;
        while (fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
        }
        if (fast) slow = slow->next;
        slow = reverse(slow);
        while (slow) {
            if (slow->val != head->val) return false;
            slow = slow->next;
            head = head->next;
        }
        return true;
    }
    
    ListNode* reverse(ListNode* cur) {
        ListNode* pre = nullptr;
        ListNode* nxt = nullptr;
        while (cur) {
            nxt = cur->next;
            cur->next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }
};