Check if Linked List is Palindrome

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

Difficulty:

JavaScript Solution

function isPalindrome(head){
  let slow = head, fast = head;
  const stack = [];
  while(fast && fast.next){
    stack.push(slow.val);
    slow = slow.next;
    fast = fast.next.next;
  }
  if(fast) slow = slow.next;
  while(slow){
    if(slow.val !== stack.pop()) return false;
    slow = slow.next;
  }
  return true;
}

Explanation: Use a stack to store first half values, then compare with the second half.

⏱ O(n) | 💾 O(n)

Java Solution

class Solution {
  public boolean isPalindrome(ListNode head){
    ListNode slow = head, fast = head;
    Stack<Integer> stack = new Stack<>();
    while(fast != null && fast.next != null){
      stack.push(slow.val);
      slow = slow.next;
      fast = fast.next.next;
    }
    if(fast != null) slow = slow.next;
    while(slow != null){
      if(slow.val != stack.pop()) return false;
      slow = slow.next;
    }
    return true;
  }
}

Explanation: Push first half into a stack, then compare with second half values for palindrome check.

⏱ O(n) | 💾 O(n)

Python Solution

def is_palindrome(head):
    slow = fast = head
    stack = []
    while fast and fast.next:
        stack.append(slow.val)
        slow = slow.next
        fast = fast.next.next
    if fast: slow = slow.next
    while slow:
        if slow.val != stack.pop(): return False
        slow = slow.next
    return True

Explanation: Compare first half (stack) with second half of list to check palindrome property.

⏱ O(n) | 💾 O(n)

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Check if Linked List is Palindrome