CodingGyan

Palindrome Check with a Deque

easy Java Collections

Problem

Print true if the array reads the same from the left and from the right, else false. Use a double-ended queue: look at its two ends and remove them together.

Constraints

  • 1 <= n <= 10^5

Examples

Input

5
1 2 3 2 1

Output

true

Solutions

Best approachTime O(n)Space O(n)

A deque lets us remove from both ends in O(1). Each round checks one matching pair. After n / 2 rounds the answer is clear.

Same code works in Java810172125Basic features only, so nothing changes between versions.
/*
 * Problem : Palindrome array.
 * Input   : n, then n integers.
 * Output  : true or false.
 * Approach: Deque.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static boolean solve(int[] a) {
        // Step 1: Put all numbers in a deque. Compare and remove the front and the back until at most one number is left.
        java.util.ArrayDeque<Integer> deque = new java.util.ArrayDeque<Integer>();
        for (int value : a) {
            deque.addLast(value);
        }
        while (deque.size() > 1) {
            if (!deque.pollFirst().equals(deque.pollLast())) {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(solve(a));
    }
}
Hints (2)
  1. What is left when n is odd?
  2. Why compare after removing both ends?