public static String toAsterisks(String password){
    //Helper method written below
    return asteriskString(password.length());
  }
  
  public static String asteriskString(int numAsterisks){
    String asterisks = "";
    for(int i=0;i<numAsterisks;i++){
      asterisks += "*";
    }
    return asterisks;
  }
  
  public static String reverse(String original){
    String reverse = "";
    for(int i=0; i<original.length(); i++){
      int oppositeIndex = original.length()-i-1;
      reverse += original.charAt(oppositeIndex);
    }
    return reverse;
  }
  
  public static boolean isPalindrome(String s){
    //Note that we only have to go up to s.length()/2 because if it's a palindrome,
    //both halves of the string will be the same.
    for(int i=0; i<s.length()/2; i++){
      int oppositeIndex = s.length()-i-1;
      //One character did not match its opposite, so we can immediately return false
      if(s.charAt(i) != s.charAt(oppositeIndex)) return false;
    }
    //We've checked all characters, and they all matched their opposite, so return true.
    return true;
  }
  
  public static String change(String nameString){
    if(nameString.indexOf(",")!=-1)
      return firstLast(nameString);
    else
      return lastFirst(nameString);
  }
  
  //Takes in a string of the form "Last, First" nad returns "First Last"
  public static String firstLast(String name){
    int commaIndex = name.indexOf(",");
    String last = name.substring(0,commaIndex);
    //We need commaIndex+1 because we don't want to include the comma, or the space.
    String first = name.substring(commaIndex+2);
    return first + " " + last;
  }
  
  //Takes in a string of the form "First Last" and returns "Last, First"
  public static String lastFirst(String name){
    int spaceIndex = name.indexOf(" ");
    String first = name.substring(0,spaceIndex);
    String last = name.substring(spaceIndex+1);
    return last + ", " + first;
  }
  
  public static void makeUppercase(char[] chars){
    for(int i=0;i<chars.length;i++){
      if(chars[i]>='a' && chars[i]<='z'){
        //How to reason about this line:
        //chars[i]-'a' is the position of the lowercase letter in the alphabet.
        //By adding 'A', we go that many steps away from capital 'A'.
        chars[i]=(char)(chars[i]-'a'+'A');
      }
    }
  }
  
  public static int[] letterFrequencies(String s){
    int[] freqs = new int[26];
    for(int i=0;i<s.length();i++){
      int letterNumber = s.charAt(i)-'A';
      freqs[letterNumber]++;
    }
    return freqs;
  }
  
  public static boolean isPrime(int n){
    for(int divisor=2;divisor<=Math.sqrt(n);divisor++){
      //n%divisor==0 means there is no remainder when we do n/divisor.
      if(n%divisor==0){
        return false;
      }
    }
    return true;
  }
  
  public static int[] buildArray(int count, int value){
    int[] arr = new int[count];
    for(int i=0;i<count;i++){
      arr[i]=value;
    }
    return arr;
  }
  
  public static void printRectangle(int rows, int cols){
    for(int i=0;i<rows;i++){
      //Helper method written above.
      System.out.println(asteriskString(cols));
    }
  }
  
  public static void printTriangle(int rows){
    for(int i=0;i<rows;i++){
      System.out.println(asteriskString(i+1));
    }
  }