Find Duplicates of array using bit array

Last Updated : 23 Feb, 2023

You have an array of N numbers, where N is at most 32,000. The array may have duplicate entries and you do not know what N is. With only 4 Kilobytes of memory available, how would print all duplicate elements in the array ?.

Examples:

Input : arr[] = {1, 5, 1, 10, 12, 10}
Output : 1 10
1 and 10 appear more than once in given
array.

Input : arr[] = {50, 40, 50}
Output : 50

Asked In: Amazon

We have 4 Kilobytes of memory which means we can address up to 8 * 4 * 2¹⁰ bits. Note that 32 * 2¹⁰ bits is greater than 32000. We can create a bit with 32000 bits, where each bit represents one integer. Note: If you need to create a bit with more than 32000 bits, then you can create easily more and more than 32000; Using this bit vector, we can then iterate through the array, flagging each element v by setting bit v to 1. When we come across a duplicate element, we print it. Below is the implementation of the idea.

Implementation:

C++

// C++ program to print all Duplicates in array
#include <bits/stdc++.h>
using namespace std;

// A class to represent an array of bits using
// array of integers
class BitArray
{
    int *arr;

    public:
    BitArray() {}

    // Constructor
    BitArray(int n)
    {
        // Divide by 32. To store n bits, we need
        // n/32 + 1 integers (Assuming int is stored
        // using 32 bits)
        arr = new int[(n >> 5) + 1];
    }

    // Get value of a bit at given position
    bool get(int pos)
    {
        // Divide by 32 to find position of
        // integer.
        int index = (pos >> 5);

        // Now find bit number in arr[index]
        int bitNo = (pos & 0x1F);

        // Find value of given bit number in
        // arr[index]
        return (arr[index] & (1 << bitNo)) != 0;
    }

    // Sets a bit at given position
    void set(int pos)
    {
        // Find index of bit position
        int index = (pos >> 5);

        // Set bit number in arr[index]
        int bitNo = (pos & 0x1F);
        arr[index] |= (1 << bitNo);
    }

    // Main function to print all Duplicates
    void checkDuplicates(int arr[], int n)
    {
        // create a bit with 32000 bits
        BitArray ba = BitArray(320000);

        // Traverse array elements
        for (int i = 0; i < n; i++)
        {
            // Index in bit array
            int num = arr[i];

            // If num is already present in bit array
            if (ba.get(num))
                cout << num << " ";

            // Else insert num
            else
                ba.set(num);
        }
    }
};

// Driver code
int main()
{
    int arr[] = {1, 5, 1, 10, 12, 10};
    int n = sizeof(arr) / sizeof(arr[0]);
    BitArray obj = BitArray();
    obj.checkDuplicates(arr, n);
    return 0;
}

// This code is contributed by
// sanjeev2552

Java

// Java program to print all Duplicates in array
import java.util.*;
import java.lang.*;
import java.io.*;

// A class to represent array of bits using
// array of integers
public class BitArray
{
    int[] arr;

    // Constructor
    public BitArray(int n)
    {
        // Divide by 32. To store n bits, we need
        // n/32 + 1 integers (Assuming int is stored
        // using 32 bits)
        arr = new int[(n>>5) + 1];
    }

    // Get value of a bit at given position
    boolean get(int pos)
    {
        // Divide by 32 to find position of
        // integer.
        int index = (pos >> 5);

        // Now find bit number in arr[index]
        int bitNo  = (pos & 0x1F);

        // Find value of given bit number in
        // arr[index]
        return (arr[index] & (1 << bitNo)) != 0;
    }

    // Sets a bit at given position
    void set(int pos)
    {
        // Find index of bit position
        int index = (pos >> 5);

        // Set bit number in arr[index]
        int bitNo = (pos & 0x1F);
        arr[index] |= (1 << bitNo);
    }

    // Main function to print all Duplicates
    static void checkDuplicates(int[] arr)
    {
        // create a bit with 32000 bits
        BitArray ba = new BitArray(320000);

        // Traverse array elements
        for (int i=0; i<arr.length; i++)
        {
            // Index in bit array
            int num  = arr[i] - 1;

            // If num is already present in bit array
            if (ba.get(num))
                System.out.print(num +" ");

            // Else insert num
            else
                ba.set(num);
        }
    }

    // Driver code
    public static void main(String[] args) throws
                              java.lang.Exception
    {
        int[] arr = {1, 5, 1, 10, 12, 10};
        checkDuplicates(arr);
    }
}

Python3

# Python3 program to print all Duplicates in array

# A class to represent array of bits using
# array of integers
class BitArray:

    # Constructor
    def __init__(self, n):

        # Divide by 32. To store n bits, we need
        # n/32 + 1 integers (Assuming int is stored
        # using 32 bits)
        self.arr = [0] * ((n >> 5) + 1)

    # Get value of a bit at given position
    def get(self, pos):

        # Divide by 32 to find position of
        # integer.
        self.index = pos >> 5

        # Now find bit number in arr[index]
        self.bitNo = pos & 0x1F

        # Find value of given bit number in
        # arr[index]
        return (self.arr[self.index] & 
                   (1 << self.bitNo)) != 0

    # Sets a bit at given position
    def set(self, pos):

        # Find index of bit position
        self.index = pos >> 5

        # Set bit number in arr[index]
        self.bitNo = pos & 0x1F
        self.arr[self.index] |= (1 << self.bitNo)

# Main function to print all Duplicates
def checkDuplicates(arr):

    # create a bit with 32000 bits
    ba = BitArray(320000)

    # Traverse array elements
    for i in range(len(arr)):

        # Index in bit array
        num = arr[i]

        # If num is already present in bit array
        if ba.get(num):
            print(num, end = " ")

        # Else insert num
        else:
            ba.set(num)

# Driver Code
if __name__ == "__main__":
    arr = [1, 5, 1, 10, 12, 10]
    checkDuplicates(arr)

# This code is contributed by
# sanjeev2552

// C# program to print all Duplicates in array
 
// A class to represent array of bits using 
// array of integers 
using System;

class BitArray 
{ 
    int[] arr; 

    // Constructor 
    public BitArray(int n) 
    { 
        // Divide by 32. To store n bits, we need 
        // n/32 + 1 integers (Assuming int is stored 
        // using 32 bits) 
        arr = new int[(int)(n >> 5) + 1]; 
    } 

    // Get value of a bit at given position 
    bool get(int pos) 
    { 
        // Divide by 32 to find position of 
        // integer. 
        int index = (pos >> 5); 

        // Now find bit number in arr[index] 
        int bitNo = (pos & 0x1F); 

        // Find value of given bit number in 
        // arr[index] 
        return (arr[index] & (1 << bitNo)) != 0; 
    } 

    // Sets a bit at given position 
    void set(int pos) 
    { 
        // Find index of bit position 
        int index = (pos >> 5); 

        // Set bit number in arr[index] 
        int bitNo = (pos & 0x1F); 
        arr[index] |= (1 << bitNo); 
    } 

    // Main function to print all Duplicates 
    static void checkDuplicates(int[] arr) 
    { 
        // create a bit with 32000 bits 
        BitArray ba = new BitArray(320000); 

        // Traverse array elements 
        for (int i = 0; i < arr.Length; i++) 
        { 
            // Index in bit array 
            int num = arr[i]; 

            // If num is already present in bit array 
            if (ba.get(num)) 
                Console.Write(num + " "); 

            // Else insert num 
            else
                ba.set(num); 
        } 
    } 

    // Driver code 
    public static void Main()
    { 
        int[] arr = {1, 5, 1, 10, 12, 10}; 
        checkDuplicates(arr); 
    } 
} 

// This code is contributed by Rajput-Ji

JavaScript

// JavaScript program to print all Duplicates in array
 
// A class to represent array of bits using
// array of integers
class BitArray {
    // Constructor
    constructor(n){
        // Divide by 32. To store n bits, we need
        // n/32 + 1 integers (Assuming int is stored
        // using 32 bits)
        this.arr = new Array((n >> 5) + 1);
    }
    
    // Get value of a bit at given position
    get(pos){
        // Divide by 32 to find position of
        // integer.
        let index = (pos >> 5);
 
        // Now find bit number in arr[index]
        let bitNo = (pos & 0x1F);
 
        // Find value of given bit number in
        // arr[index]
        let arrCopy = this.arr 
        return (arrCopy[index] & (1 << bitNo)) != 0;
    }
    // Sets a bit at given position
    set(pos){
        // Find index of bit position
        var index = (pos >> 5);
 
        // Set bit number in arr[index]
        var bitNo = (pos & 0x1F);
        var arr1 = this.arr;
        arr1[index] = arr1[index] | (1 << bitNo);
        this.arr = arr1;
    }
}
 
// Main function to print all Duplicates
function checkDuplicates(arr){
    // create a bit with 32000 bits
    var ba = new BitArray(320000);
 
    // Traverse array elements
    for (var i = 0; i < arr.length; i++) {
        // Index in bit array
        var num = arr[i];
        // If num is already present in bit array
        if (ba.get(num))
            console.log(num);
        // Else insert num
        else
            ba.set(num);
    }
}
 
// Driver code
var a = [ 1, 5, 1, 10, 12, 10 ];
checkDuplicates(a);
 
// This code is contributed by Kirti Agarwal(kirtiagarwal23121999)

Output

1 10

Time Complexity: O(N)

Space Complexity: O(1)

Find Duplicates of array using bit array

Mr. Somesh Awasthi

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Find Duplicates of array using bit array

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