Hand tracing the code, this is java code.

public int mystery(String str) {

if (str.length() == 0)

return 0;

if (str.charAt(0) == 'x')

return 1 + mystery(str.substring(1));

return mystery(str.substring(1));

}

mystery(xaxbxcx)

public int foo(String str) {

if (str.length() < 2)

return 0;

if (str.substring(0,2).equals("hi"))

return 1 + foo(str.substring(1));

else

return foo(str.substring(1));

}

}

foo(xhixhy)

public static String deez(String str) {

if ((null == str) || (str.length() <= 1)) {

return str;

}

return deez(str.substring(1)) + str.charAt(0);

}

deez(track)