LeetCode-in-All
234. Palindrome Linked List
Easy
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
%% Definition for singly-linked list.
%%
%% -record(list_node, {val = 0 :: integer(),
%% next = null :: 'null' | #list_node{}}).
-spec is_palindrome(Head :: #list_node{} | null) -> boolean().
-import(lists,[reverse/1]).
list_to_seq(null) -> [];
list_to_seq(Head) ->
[Head#list_node.val] ++ list_to_seq(Head#list_node.next).
is_palindrome(null) -> false;
is_palindrome(Head) ->
Seq = list_to_seq(Head),
Seq =:= reverse(Seq).