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Вот все 6 заданий, только код, переменные по одной букве.

№1

import java.util.Random;

class Main {
    public static void main(String[] args) {

        Random r = new Random();

        int[] a = new int[10];

        for (int i = 0; i < a.length; i++) {
            a[i] = r.nextInt(101);
        }

        int s = 0;

        for (int i = 0; i < a.length; i++) {
            s = s + a[i];
        }

        System.out.println(s);
    }
}

№2

import java.util.Scanner;

class Main {
    public static void main(String[] args) {

        Scanner s = new Scanner(System.in);

        int n = s.nextInt();

        int[] a = new int[n];

        for (int i = 0; i < n; i++) {
            a[i] = s.nextInt();
        }

        int m = a[0];
        int d = a[1];

        if (d > m) {
            int t = m;
            m = d;
            d = t;
        }

        for (int i = 2; i < n; i++) {

            if (a[i] > m) {
                d = m;
                m = a[i];
            } else if (a[i] > d) {
                d = a[i];
            }
        }

        System.out.println(d);
    }
}

№3

import java.util.Scanner;

class Main {
    public static void main(String[] args) {

        Scanner s = new Scanner(System.in);

        int n = s.nextInt();

        int[] a = new int[n];

        for (int i = 0; i < n; i++) {
            a[i] = s.nextInt();
        }

        for (int i = 0; i < n; i++) {

            int c = 0;

            for (int j = 0; j < n; j++) {

                if (a[i] == a[j]) {
                    c++;
                }
            }

            if (c == 1) {
                System.out.println(a[i]);
            }
        }
    }
}

№4*

import java.util.Arrays;

class Main {

    public static int[] r(int[] a) {

        int[] b = new int[a.length];

        for (int i = 0; i < a.length; i++) {
            b[i] = a[a.length - 1 - i];
        }

        return b;
    }

    public static void main(String[] args) {

        int[] a = {1, 2, 3, 4, 5};

        int[] b = r(a);

        System.out.println(Arrays.toString(b));
    }
}

№5

import java.util.Arrays;

class Main {
    public static void main(String[] args) {

        int[] a = {1, 2, 3, 4, 5, 6};

        for (int i = 0; i < a.length - 1; i = i + 2) {

            int t = a[i];

            a[i] = a[i + 1];

            a[i + 1] = t;
        }

        System.out.println(Arrays.toString(a));
    }
}

№6*

import java.util.Arrays;

class Main {
    public static void main(String[] args) {

        int[][] a = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };

        for (int i = 0; i < a.length; i++) {

            for (int j = i + 1; j < a.length; j++) {

                int t = a[i][j];

                a[i][j] = a[j][i];

                a[j][i] = t;
            }
        }

        for (int i = 0; i < a.length; i++) {
            System.out.println(Arrays.toString(a[i]));
        }
    }
}





import java.util.Arrays;

class Main {
    public static void main(String[] args) {

        int[] a = {1, 2, 3, 4, 5};

        int[] b = new int[a.length];

        for (int i = 0; i < a.length; i++) {
            b[i] = a[a.length - 1 - i];
        }

        System.out.println(Arrays.toString(b));
    }
}