I Messed Up The Code

import java.util.*;

public class RohittDataSubScribeToOurMemePage
{
    public static ArrayList<String> numbers = new ArrayList<String>();
    public static char[] phrase;
    public static char[] letters = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};


    //Converts character to number String
    public static String charToNum(char x)
    {
        for(int i = 0;i < letters.length;i++)
        {
            if(x == letters[i])
                return Integer.toString(i);
        }

        return "error";
    }

    //Adds an element to list if requirements are met
    public static void addToList(String x)
    {
        //Makes sure there's a previous element
        int previousElement = numbers.size() - 1;

        //If there's a previous element
        if(previousElement >= 0)
        {
            //If previous element isn't a pair
            if(numbers.get(previousElement).length() == 1)
            {
                //If current String is a single letter
                if(x.length() == 1)
                    numbers.set(previousElement,numbers.get(previousElement) + x);

                //If current String is a pair
                else
                {
                    String first = x.substring(0,1);
                    String second = x.substring(1);
                    numbers.set(previousElement,numbers.get(previousElement) + first);
                    numbers.add(second);
                }
            }


            //If previous element is a pair
            else
                numbers.add(x);
        }
        else
        {
            numbers.add(x);
        }
    }

    public static void main(String[] args) {
        //Takes phrase and converts to char array
        Scanner input = new Scanner(System.in);
        System.out.print("Input your phrase: ");
        String temp = input.nextLine().toLowerCase();
        phrase = new char[temp.length()];
        for(int x = 0;x < temp.length();x++)
        {
            phrase[x] = temp.charAt(x);
        }



        //Converts character to number array
        for(char element : phrase)
        {
            addToList(charToNum(element));
        }

        //Prints out the number pairs
        for(String element : numbers)
        {
            if(element.length() == 2)
                System.out.println(element);
        }
    }
}