234. Palindrome Linked List

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LeetCode

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    private ListNode endOfFirstHalf(ListNode head){
        ListNode fast=head,slow=fast;
        while(fast.next!=null&&fast.next.next!=null){
            fast=fast.next.next;
            slow=slow.next;
        }
        return slow;
    }
    private ListNode reverseList(ListNode head){
        ListNode prev=null,curr=head;
        while(curr!=null){
            ListNode nextNode=curr.next;
            curr.next=prev;
            prev=curr;
            curr=nextNode;
        }
        return prev;
    }
    public boolean isPalindrome(ListNode head) {
        if(head==null||head.next==null) return true;
        
        ListNode endOfFirstHalfNode=endOfFirstHalf(head);
        ListNode reversedSecondHalf=reverseList(endOfFirstHalfNode.next);
        
        ListNode p1=head,p2=reversedSecondHalf;
        while(p2!=null&&p1.val==p2.val){
            p1=p1.next;
            p2=p2.next;
        }
        return p2==null;
    }
}

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