Problems - Set 1

Easy

1. Addition of Two Numbers

public static int add(int a, int b) {
        while (b != 0) {
            int carry = a & b; // Calculate the carry by performing bitwise AND
            a = a ^ b; // Perform bitwise XOR to add the numbers without carry
            b = carry << 1; // Left shift the carry to perform addition with the next bit
        }
        return a;
}

public static void main(String[] args) {
        int num1 = 2;
        int num2 = 3;
        int sum = add(num1, num2);
        System.out.println("Sum of " + num1 + " and " + num2 + " is: " + sum);
}

The provided code snippet implements binary addition without using the built-in + operator.

1. Identifying Bits Set to 1:

• Bitwise AND compares the corresponding bits of a and b.

• It returns 1 only if the bits in both positions of a and b are 1. Otherwise, it returns 0.

1. Significance in Carry Calculation:

• In binary addition, a carry is generated when adding two bits that are both 1 (e.g., 1 + 1 = 10, where 1 is the carry bit).

• The carry = a & b; line essentially identifies the positions where both a and b have bits set to 1. These positions will generate a carry bit during the addition.

Example:

Let's consider a = 5 (binary: 101) and b = 3 (binary: 011).

• a & b = 001. Here, only the least significant bit (LSB) position has 1s in both a and b, indicating a carry will be generated for this bit position.

1. Using Carry for Addition:

• The subsequent line a = a ^ b; performs bitwise XOR (^) on a and b. XOR is 1 only when the corresponding bits are different.

• This line effectively adds a and b without considering the carry for the current bit position. The carry will be addressed in the next iteration of the loop.

1. Carry bit is shifted left

These carry bits are then shifted left in the next line (b = carry << 1;) to be added in the subsequent bit positions (next bit position) of a and b.

2. Swapping the 2 numbers

public static void main(String[] args) {
    int a = 2;
    int b = 3;
    System.out.println("Number before swap: a = " + a + " b = " + b);
    // Logic of XOR operator
    a = a ^ b;
    b = a ^ b;
    a = a ^ b;
    System.out.println("Number after swap: a = " + a + " b = " + b);
}

3. Convert Decimal to Binary Conversion

    // Function that convert Decimal to binary 
    public void decToBinary(int n) 
    { 
        // Size of an integer is assumed to be 32 bits 
        for (int i = 31; i >= 0; i--) { 
            int k = n >> i; 
            if ((k & 1) > 0) 
                System.out.print("1"); 
            else
                System.out.print("0"); 
        } 
    } 

By iterating through each bit position and checking its value using the right shift and bitwise AND operations, the code determines whether each bit in the binary representation of n is 0 or 1.

4. Check if a Number is Power of Two

Given a number n, check if it is a power of two.

Input: n = 8
Output: true

Input: n = 7
Output: false

Approach

A number is a power of two if it has only one bit set to 1 in its binary representation. For example:

• 1 → 0001 ( 2^0 )

• 2 → 0010 ( 2^1 )

• 4 → 0100 ( 2^2 )

• 8 → 1000 ( 2^3 )

Let's take n=8 (1000 in binary).

• n−1=7 (0111 in binary)

• n&(n−1) = 1000&0111 = 0000

For a non-power of two, like n=6 (0110 in binary):

• n−1=5 (0101 in binary)

• n&(n−1) = 0110&0101 = 0100≠0

public class PowerOfTwo {
    public static boolean isPowerOfTwo(int n) {
        return n > 0 && (n & (n - 1)) == 0;
    }

    public static void main(String[] args) {
        System.out.println(isPowerOfTwo(8)); // true
        System.out.println(isPowerOfTwo(7)); // false
    }
}

5. Count the Number of Set Bits (Hamming Weight)

Given an integer, count the number of 1s in its binary representation.

Input: n = 9  (1001)
Output: 2

Approach: Use n & (n - 1) to remove the lowest set bit in a loop.

public class CountSetBits {
    public static int countSetBits(int n) {
        int count = 0;
        while (n > 0) {
            count++;
            n = n & (n - 1); // Remove the last set bit
        }
        return count;
    }

    public static void main(String[] args) {
        System.out.println(countSetBits(9)); // 2
    }
}

6. Find the Single Non-Repeating Element (XOR Trick)

Given an array where every element appears twice except for one, find the single element.

Input: [4, 3, 2, 4, 2, 3, 5]
Output: 5

Approach:

• XORing a number with itself results in 0 (a ^ a = 0).

• XORing all numbers cancels out the duplicates, leaving only the unique number.

public class SingleNumber {
    public static int findSingleNumber(int[] nums) {
        int res = 0;
        for (int num : nums) {
            res ^= num;
        }
        return res;
    }

    public static void main(String[] args) {
        int[] nums = {4, 3, 2, 4, 2, 3, 5};
        System.out.println(findSingleNumber(nums)); // 5
    }
}

Medium

1. Reverse Bits of an Integer

Given a 32-bit integer, reverse its bits.

Input:  00000010100101000001111010011100
Output: 00111001011110000010100101000000

Approach:

• Iterate through the bits, shifting the result left and adding the last bit of n to it.

Java Solution:

public class ReverseBits {
    public static int reverseBits(int n) {
        int result = 0;
        for (int i = 0; i < 32; i++) {
            result <<= 1; // Shift left
            result |= (n & 1); // Add the last bit of n
            n >>= 1; // Shift right n
        }
        return result;
    }

    public static void main(String[] args) {
        int n = 0b00000010100101000001111010011100;
        System.out.println(Integer.toBinaryString(reverseBits(n)));
    }
}

2. Find the Missing Number

Problem Statement:

Given an array of size n containing numbers from 0 to n with one missing, find the missing number.

Input: [3, 0, 1]
Output: 2

Approach: Use XOR: missing = (0 ^ 1 ^ 2 ^ ... ^ n) ^ (arr[0] ^ arr[1] ^ ... ^ arr[n-1])

public class MissingNumber {
    public static int findMissingNumber(int[] nums) {
        int n = nums.length;
        int xor = 0;

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

        for (int num : nums) {
            xor ^= num;
        }

        return xor;
    }

    public static void main(String[] args) {
        int[] nums = {3, 0, 1};
        System.out.println(findMissingNumber(nums)); // 2
    }
}

Difficult