Add implementations for Exponentiation, Fibonacci, Quicksort, CrossTotal, Palindrome, and RomanNumber classes

src/de/dhbwka/java/exercise/methods/Exponentiation.java

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+package de.dhbwka.java.exercise.methods;
+
+public class Exponentiation {
+  public static double xPowerN(double x, int n) {
+    if (n == 0)
+      return 1;
+    if (n > 0)
+      return x * xPowerN(x, n - 1);
+    return 1 / xPowerN(x, -n);
+  }
+
+  public static void main(String[] args) {
+    System.out.println(xPowerN(2, 3));
+    System.out.println(xPowerN(2, -3));
+  }
+}

src/de/dhbwka/java/exercise/methods/Fibonacci.java

@@ -0,0 +1,17 @@
+package de.dhbwka.java.exercise.methods;
+
+public class Fibonacci {
+  public static long fib(int n) {
+    if (n == 0)
+      return 0;
+    if (n == 1)
+      return 1;
+    return fib(n - 1) + fib(n - 2);
+  }
+
+  public static void main(String[] args) {
+    for (int i = 0; i < 10; i++) {
+      System.out.println(fib(i));
+    }
+  }
+}

src/de/dhbwka/java/exercise/methods/Quicksort.java

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+package de.dhbwka.java.exercise.methods;
+
+import java.util.Arrays;
+
+public class Quicksort {
+  public static void sort(int[] array) {
+    quicksort(array, 0, array.length - 1);
+  }
+
+  public static void quicksort(int[] array, int left, int right) {
+    if (left < right) {
+      int pivot = partition(array, left, right);
+      quicksort(array, left, pivot - 1);
+      quicksort(array, pivot + 1, right);
+    }
+  }
+
+  public static int partition(int[] array, int left, int right) {
+    int i = left;
+    int j = right - 1;
+    int pivot = array[right];
+
+    do {
+      while (array[i] <= pivot && i < right) {
+        i++;
+      }
+      while (array[j] >= pivot && j > left) {
+        j--;
+      }
+      if (i < j) {
+        int temp = array[i];
+        array[i] = array[j];
+        array[j] = temp;
+      }
+    } while (i < j);
+
+    if (array[i] > pivot) {
+      int temp = array[i];
+      array[i] = pivot;
+      array[right] = temp;
+    }
+
+    return i;
+  }
+
+  public static void main(String[] args) {
+    int[] array = { 3, 6, 8, 10, 1, 2, 1 };
+    sort(array);
+    System.out.print("Sorted array: " + Arrays.toString(array));
+  }
+}

src/de/dhbwka/java/exercise/strings/CrossTotal.java

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+package de.dhbwka.java.exercise.strings;
+
+public class CrossTotal {
+  public static int crossTotal(String number) {
+    int sum = 0;
+    for (int i = 0; i < number.length(); i++) {
+      char c = number.charAt(i);
+      if (c >= '0' && c <= '9') {
+        sum += c - '0';
+      }
+    }
+    return sum;
+  }
+
+  public static void main(String[] args) {
+    System.out.println(crossTotal("12345"));
+    System.out.println(crossTotal("1a2b3c4d5e"));
+  }
+}

src/de/dhbwka/java/exercise/strings/Palindrome.java

@@ -0,0 +1,23 @@
+package de.dhbwka.java.exercise.strings;
+
+public class Palindrome {
+  public static String reverse(String s) {
+    String reversed = "";
+    for (int i = s.length() - 1; i >= 0; i--) {
+      reversed += s.charAt(i);
+    }
+    return reversed;
+  }
+
+  public static boolean isPalindrome(String s) {
+    return s.equals(reverse(s));
+  }
+
+  public static void main(String[] args) {
+    System.out.println(isPalindrome("anna"));
+    System.out.println(isPalindrome("otto"));
+    System.out.println(isPalindrome("lagerregal"));
+    System.out.println(isPalindrome("rentner"));
+    System.out.println(isPalindrome("hallo"));
+  }
+}

src/de/dhbwka/java/exercise/strings/RomanNumber.java

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+package de.dhbwka.java.exercise.strings;
+
+public class RomanNumber {
+  public static int romanToInt(String roman) {
+    roman = roman.replaceAll("IV", "IIII").replaceAll("IX", "VIIII").replaceAll("XL", "XXXX").replaceAll("XC", "LXXXX")
+        .replaceAll("CD", "CCCC").replaceAll("CM", "DCCCC");
+
+    int sum = 0;
+    for (int i = 0; i < roman.length(); i++) {
+      char c = roman.charAt(i);
+      switch (c) {
+        case 'I':
+          sum += 1;
+          break;
+        case 'V':
+          sum += 5;
+          break;
+        case 'X':
+          sum += 10;
+          break;
+        case 'L':
+          sum += 50;
+          break;
+        case 'C':
+          sum += 100;
+          break;
+        case 'D':
+          sum += 500;
+          break;
+        case 'M':
+          sum += 1000;
+          break;
+      }
+    }
+    return sum;
+  }
+
+  public static void main(String[] args) {
+    System.out.println(romanToInt("I"));
+    System.out.println(romanToInt("II"));
+    System.out.println(romanToInt("III"));
+    System.out.println(romanToInt("IV"));
+    System.out.println(romanToInt("V"));
+    System.out.println(romanToInt("VI"));
+    System.out.println(romanToInt("MCMXCIV"));
+    System.out.println(romanToInt("MMXXI"));
+  }
+}