Java Programs for Practice

Swap Two Numbers Without Using a Third Variable

Trick:

Use arithmetic operators.

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

        int a = 10;
        int b = 20;

        a = a + b;
        b = a - b;
        a = a - b;

        System.out.println("a = " + a);
        System.out.println("b = " + b);
    }
}

Output

a = 20
b = 10

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2. Find Largest of Three Numbers Without Using if-else

Trick:

Use the ternary operator.

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

        int a = 15;
        int b = 30;
        int c = 25;

        int largest = (a > b) ? (a > c ? a : c)
                              : (b > c ? b : c);

        System.out.println("Largest = " + largest);
    }
}

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3. Reverse a Number

Trick:

Extract the last digit using % 10.

public class ReverseNumber {

    public static void main(String[] args) {

        int num = 12345;
        int reverse = 0;

        while(num != 0)
        {
            int digit = num % 10;
            reverse = reverse * 10 + digit;
            num = num / 10;
        }

        System.out.println(reverse);
    }
}

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4. Count Digits Without Converting to String

Trick:

Repeatedly divide by 10.

public class CountDigits {

    public static void main(String[] args) {

        int num = 987654;
        int count = 0;

        while(num != 0)
        {
            count++;
            num /= 10;
        }

        System.out.println("Digits = " + count);
    }
}

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5. Check Palindrome Number

Trick:

Reverse the number and compare.

public class Palindrome {

    public static void main(String[] args) {

        int num = 121;
        int original = num;
        int reverse = 0;

        while(num != 0)
        {
            reverse = reverse * 10 + num % 10;
            num /= 10;
        }

        if(original == reverse)
            System.out.println("Palindrome");
        else
            System.out.println("Not Palindrome");
    }
}

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6. Find Missing Number in an Array

Trick:

Use the formula:$$n(n+1)/2$$n(n+1)/2

public class MissingNumber {

    public static void main(String[] args) {

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

        int n = 5;

        int expected = n * (n + 1) / 2;

        int sum = 0;

        for(int i : arr)
            sum += i;

        System.out.println("Missing = " + (expected - sum));
    }
}

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7. Find Duplicate Elements

Trick:

Nested loops (easy version).

public class DuplicateElements {

    public static void main(String[] args) {

        int arr[] = {2,5,6,2,7,8,5};

        for(int i=0;i<arr.length;i++)
        {
            for(int j=i+1;j<arr.length;j++)
            {
                if(arr[i]==arr[j])
                    System.out.println(arr[i]);
            }
        }
    }
}

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8. Find Second Largest Number

Trick:

Maintain largest and secondLargest together.

public class SecondLargest {

    public static void main(String[] args) {

        int arr[] = {10,50,30,90,80};

        int largest = Integer.MIN_VALUE;
        int second = Integer.MIN_VALUE;

        for(int num : arr)
        {
            if(num > largest)
            {
                second = largest;
                largest = num;
            }
            else if(num > second && num != largest)
            {
                second = num;
            }
        }

        System.out.println("Second Largest = " + second);
    }
}

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9. Check Whether Two Strings are Anagrams

Trick:

Sort both strings.

import java.util.Arrays;

public class Anagram {

    public static void main(String[] args) {

        String s1 = "listen";
        String s2 = "silent";

        char a[] = s1.toCharArray();
        char b[] = s2.toCharArray();

        Arrays.sort(a);
        Arrays.sort(b);

        if(Arrays.equals(a,b))
            System.out.println("Anagram");
        else
            System.out.println("Not Anagram");
    }
}

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10. Count Frequency of Every Character

Trick:

Use an array of size 256.

public class CharacterFrequency {

    public static void main(String[] args) {

        String str = "programming";

        int freq[] = new int[256];

        for(int i=0;i<str.length();i++)
        {
            freq[str.charAt(i)]++;
        }

        for(int i=0;i<256;i++)
        {
            if(freq[i] != 0)
            {
                System.out.println((char)i + " : " + freq[i]);
            }
        }
    }
}