CS 256 - Recursion Problems

1. Write a recursive function reverse(str) to reverse a string.

   String reverse(String str) {                
     if (str.length() <= 1)
        return str;
     else
        return reverse(str.substring(1)) + str.charAt(0);
   }
   

2. Write a recursive function contains(str,ch) that returns true if the string str contains the character ch. Return false otherwise.

     
   boolean contains(String str, char ch) {
     // base case, the empty string does not contain ch
     if (str.length() == 0)
        return false;
           
     // check if the first character is ch
     else if (str.charAt(0) == ch)
        return true;
           
     // otherwise look in the rest of the string, recursively
     else
        return contains(str.substring(1), ch);
   }
   

3. Write a recursive function to sum the number of digits in an integer. For example, sum(7128) should return 18.

   int sum_digits(int x) {
     if  (x == 0) 
       return 0;
     else
       return (x % 10) + sum_digits(x/10);
   }
   

4. Self check 1,6 on page 251.
5. Self check 1 page 259.
6. Self check 1,2 page 267.

For answers to the self check exercises see the solutions manual in the QRC.