Palindrome Linked List

Slow and Fast Pointer

Posted by Micah on November 2, 2018



C++ solution. Use slow and fast pointer, during the traversal: slow pointer takes one step and fast takes two steps slow pointer reverse its next to point to its prev when fast pointer reache the end of the linked list, slow will reach the middle Traverse the first and second half, compare each node value. if all equal, return true, otherwise return false time complexity: O(n), space compexity: O(1)

C++ Solution

class Solution {
    bool isPalindrome(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return true;
        ListNode* prev = NULL;
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast != NULL && fast->next != NULL){
            fast = fast->next->next;
            ListNode* next = slow->next;
            slow->next = prev;
            prev = slow;
            slow = next;
        if(fast != NULL){
            slow = slow->next;
        while(slow != NULL && prev != NULL){
            if(slow->val != prev->val)
                return false;
            slow = slow->next;
            prev = prev->next;
        return true;