C Sc 127B: 20 Recursion Questions
For Quiz on 30-March-2007
Do as many as you can before section. Writing many recursive solutions will help you understand recursion. Some
of these exercises may be a repeat of what you have seen or done, but now write them by hand to practice for the
quiz on recursion, 30-March-2007
1. Write the output generated by the method call mystery("X", 6);
public void mystery(String s, int digit) {
if(digit <= 1)
System.out.println(digit);
else {
s = s + "<";
mystery(s, digit - 2);
System.out.println(s + digit);
}
}
2. Write the return values from each call to the method named mystery.
____mystery(0) ____mystery(1) ____mystery(2) ____mystery(3) ____mystery(4)
public int mystery(int n) {
if (n <= 0)
return 1;
else
return 3 + mystery(n - 1);
}
3. Write the return values from each call to the method named mysteryTwo.
________ mysteryTwo("T") ________ mysteryTwo("ab") ________ mysteryTwo("123")
public String mysteryTwo(String s) {
if (s.length() == 0)
return "";
else
return mysteryTwo(s.substring(1, s.length())) + "/" + s.charAt(0);
}
4. Write the output of stars with arguments 1, 2, and 3
public static void stars(int n) {
if (n > 1)
stars(n-1);
for (int i = 0; i < n; i++)
System.out.print("*");
System.out.println();
}
5. Write recursive method goingUp so it displays all the numbers from the first argument to the last in
ascending order (separate with a space). Use recursion, do NOT use a loop.
goingUp(1, 5); // 1 2 3 4 5
goingUp(2, 7); // 2 3 4 5 6 7
goingUp(3, 3); // 3
6. Implement the Greatest Common Divisor algorithm as recursive method GCD. Use recursion. Do NOT
use a loop.
0)%,(
0
),( nifnmnGCD
nifm
nmGCD
1
