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                                                                Chapter2:Recursion

-- Printing odd or even number with recursion:

public static void odd(int a, int b) {
        if (a == b) return;
        if (a % 2 == 1)
            System.out.println(a);
        odd(++a, b);
}

15.How to write recursion in 3 steps?

package org.example.recursion;

public class Main {
    public static void main(String[] args) {
        System.out.println(factorial1(5));
        System.out.println(factorial2(5));
        System.out.println(factorial3(5));
    }

    public static Integer factorial1(Integer n) {
        return n == 0 ? 1 : n * factorial1(n - 1);
    }

    public static Integer factorial2(Integer n) {
        return n < 0 ? -1 : n == 0 ? 1 : n * factorial2(n - 1);
    }

    public static Integer factorial3(Integer n) {
        if (n < 0) return -1;
        if (n == 0 || n == 1) return 1;
        return n * factorial3(n - 1);
    }
}

16.Fibonacci using recursion

package org.example.recursion;

public class Main {
    public static void main(String[] args) {
        System.out.println(fibonacci(7));
    }

    public static int fibonacci(int n) {
        if (n < 0) return -1;
        if (n == 0 || n == 1) return n;
        return fibonacci(n - 1) + fibonacci(n - 2);
    }
}

18.Question 1 - Sum of digits

package org.example.recursion;

public class Main {
    public static void main(String[] args) {
        System.out.println(sumOfDigits(123));
    }

    public static Integer sumOfDigits(Integer n) {
        if (n == 0 || n < 0) return 0;
        return n % 10 + sumOfDigits(n / 10);
    }
}

19.Question 2 - Power

package org.example.recursion;

public class Main {
    public static void main(String[] args) {
        System.out.println(power(3, 4));
    }

    public static Integer power(Integer number, Integer power) {
        if (power < 1) return 1;
        if (number < 1) return -1;
        return number * power(number, --power);
    }
}

20.Question 3 - GCD(Greatest Common Divisor)

-- Euclidian algorithm

package org.example.recursion;

public class Main {
    public static void main(String[] args) {
        System.out.println(gcd(48, 12));
    }

    public static Integer gcd(Integer a, Integer b) {
        if (a < 0 || b < 0) return -1;
        if (b == 0) return a;
        return gcd(b, a % b);
    }
}

-- Euclidian algorithm without recursion

package org.example.recursion;

public class Main {
    public static void main(String[] args) {
        System.out.println(gcd2(12, 33));
    }

    public static Integer gcd(Integer a, Integer b) {
        if (a > b) {
            a += b;
            b = a - b;
            a = a - b;
        }
        int remainder = b % a;
        while (remainder != 0) {
            b = a;
            a = remainder;
            remainder = b % a;
        }
        return a;
    }

    public static Integer gcd2(Integer a, Integer b) {
        if (b > a) {
            a += b;
            b = a - b;
            a = a - b;
        }
        while (b != 0) {
            int remainder = a % b;
            a = b;
            b = remainder;
        }
        return a;
    }
}

21.Question4 - Decimal to Binary

public static Integer binaryRecursion(Integer n) {
    if (n < 0) return null;
    if (n == 0) return 0;
    return (n % 2) + 10 * binaryRecursion(n / 2);
}

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1. Interface Iterable.

package org.example.part1;

import java.util.Iterator;

public class Counter implements Iterable<Integer> {
    private int count;

    public Counter(int count) {
        this.count = count;
    }

    public Iterator<Integer> iterator() {
        return new Iterator<Integer>() {
            private int i = 0;

            @Override
            public boolean hasNext() {
                return i < count;
            }

            @Override
            public Integer next() {
                i++;
                return i;
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }

} 

package org.example;

import org.example.part1.Counter;

public class Main {
    public static void main(String[] args) {
        int total = 0;
        for (int i : new Counter(3)) {
            total += i;
        }
        System.out.println(total);
    }
}

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package az.etibarli.test8;

public class MyArrayList {
    private String[] arr = new String[10];
    int size = 0;

    public void add(String element) {
        ensureCapacity();
        arr[size++] = element;
    }

    public void ensureCapacity() {
        if (arr.length == size) {  // full
            String[] arr2 = new String[arr.length / 2 + arr.length];
            for (int i = 0; i < arr.length; i++) {
                arr2[i] = arr[i];
            }
            arr = arr2;
        }
    }

    public String get(int i) {
        return arr[i];
    }

    public int size() {
        return size;
    }
}