234. Palindrome Linked List

Description of Problem

Given the head of a singly linked list, return true if it is a palindrome or false otherwise.

Example 1:

Input: head = [1,2,2,1]
Output: true

Example 2:

Input: head = [1,2]
Output: false

Constraints:

• The number of nodes in the list is in the range [1, 105].
• 0 <= Node.val <= 9

Follow up: Could you do it in O(n) time and O(1) space?

Solution

Tags: LinkedList

Explanation

If you solve [143. Reorder List], it is not difficult to re-use the logic.

Code (C++)

/**
 * 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* fast = head;
        ListNode* slow = head;

        // 1. Find the middle node
        while(fast->next != nullptr && fast->next->next != nullptr){
            slow = slow->next;
            fast = fast->next->next;
        }

        // 2. Cut and Reverse
        ListNode* curr = slow->next; 
        ListNode* prev = nullptr;
        slow->next = nullptr; // Cut

        // Reverse
        while(curr != nullptr){
            ListNode* temp = curr;
            curr = temp->next;
            temp->next = prev;
            prev = temp;
        }

        // 3. Merge
        ListNode* list1 = head; ListNode* list2 = prev;
        while(list1 != nullptr && list2 != nullptr){
            if (list1->val != list2->val){
                return false;
            }
            list1 = list1->next;
            list2 = list2->next;
        }

        return true;
    }
};

Complexity

Time complexity:

• \(T(n)=O(n)\)
  • There are only nearly constant number of n iterations

Auxiliary Space:

• \(S(n)=O(1)\)
  • Use only constant number of variables