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Length of the longest subarray whose Bitwise XOR is K
  • Difficulty Level : Basic
  • Last Updated : 30 Dec, 2020
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Given an array arr[] of size N and an integer K, the task is to find the length of the longest subarray having Bitwise XOR of all its elements equal to K.

Examples:

Input: arr[] = { 1, 2, 4, 7, 2 }, K = 1
Output: 3
Explanation: 
Subarray having Bitwise XOR equal to K(= 1) are { { 1 }, { 2, 4, 7 }, { 1 } }.
Therefore, the length of longest subarray having bitwise XOR equal to K(= 1) is 3

Input: arr[] = { 2, 5, 6, 1, 0, 3, 5, 6 }, K = 4
Output: 6
Explanation:
Subarray having Bitwise XOR equal to K(= 4) are { { 6, 1, 0, 3 }, { 5, 6, 1, 0, 3, 5 } }.
Therefore, the length of longest subarray having bitwise XOR equal to K(= 4) is 6.

Approach: The problem can be solved using Hashing and Prefix Sum technique. Following are the observation:



a1 ^ a2 ^ a3 ^ ….. ^ an = K

=> a2 ^ a3 ^ ….. ^ an ^ K = a1

Follow the steps below to solve the problem:

  • Initialize a variable, say prefixXOR, to store the Bitwise XOR of all elements up to the ith index of the given array.
  • Initialize a Map, say mp, to store the indices of the computed prefix XORs of the array.
  • Initialize a variable, say maxLen, to store the length of the longest subarray whose Bitwise XOR is equal to K.
  • Traverse the array arr[] using variable i. For every ith index, update prefixXOR = prefixXOR ^ arr[i] and check if (prefixXOR ^ K) is present in the Map or not. If found to be true, then update maxLen = max(maxLen, i – mp[prefixXOR ^ K]).
  • If prefixXOR is not present in the Map, then insert prefixXOR into the Map.
  • Finally, print the value of maxLen.

Below is the implementation of the above approach:

C++




// C++ program to implement
// the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the length of the longest
// subarray whose bitwise XOR is equal to K
int LongestLenXORK(int arr[], int N, int K)
{
     
    // Stores prefix XOR
    // of the array
    int prefixXOR = 0;
 
    // Stores length of longest subarray
    // having bitwise XOR equal to K
    int maxLen = 0;
 
    // Stores index of prefix
    // XOR of the array
    map<int, int> mp;
     
     
    // Insert 0 into the map
    mp[0] = -1;
 
    // Traverse the array
    for (int i = 0; i < N; i++) {
 
        // Update prefixXOR
        prefixXOR ^= arr[i];
 
        // If (prefixXOR ^ K) present
        // in the map
        if (mp.count(prefixXOR ^ K)) {
 
            // Update maxLen
            maxLen = max(maxLen,
               (i - mp[prefixXOR ^ K]));
        }
         
        // If prefixXOR not present
        // in the Map
        if (!mp.count(prefixXOR)) {
 
            // Insert prefixXOR
            // into the map
            mp[prefixXOR] = i;
        }
    }
     
    return maxLen;
}
 
// Driver Code
int main()
{
    int arr[] = { 1, 2, 4, 7, 2 };
    int N = sizeof(arr) / sizeof(arr[0]);
    int K = 1;
    cout<< LongestLenXORK(arr, N, K);
 
    return 0;
}

Java




// Java program to implement
// the above approach
import java.util.*;
 
class GFG{
 
// Function to find the length of the longest
// subarray whose bitwise XOR is equal to K
static int LongestLenXORK(int arr[],
                          int N, int K)
{
     
    // Stores prefix XOR
    // of the array
    int prefixXOR = 0;
 
    // Stores length of longest subarray
    // having bitwise XOR equal to K
    int maxLen = 0;
 
    // Stores index of prefix
    // XOR of the array
    HashMap<Integer,
            Integer> mp = new HashMap<Integer,
                                      Integer>();
                                       
    // Insert 0 into the map
    mp.put(0, -1);
 
    // Traverse the array
    for(int i = 0; i < N; i++)
    {
         
        // Update prefixXOR
        prefixXOR ^= arr[i];
 
        // If (prefixXOR ^ K) present
        // in the map
        if (mp.containsKey(prefixXOR ^ K))
        {
             
            // Update maxLen
            maxLen = Math.max(maxLen,
               (i - mp.get(prefixXOR ^ K)));
        }
         
        // If prefixXOR not present
        // in the Map
        if (!mp.containsKey(prefixXOR))
        {
             
            // Insert prefixXOR
            // into the map
            mp.put(prefixXOR, i);
        }
    }
    return maxLen;
}
 
// Driver Code
public static void main(String[] args)
{
    int arr[] = { 1, 2, 4, 7, 2 };
    int N = arr.length;
    int K = 1;
     
    System.out.print(LongestLenXORK(arr, N, K));
}
}
 
// This code is contributed by Amit Katiyar

Python3




# Python3 program to implement
# the above approach
 
# Function to find the length of the longest
# subarray whose bitwise XOR is equal to K
def LongestLenXORK(arr, N, K):
     
    # Stores prefix XOR
    # of the array
    prefixXOR = 0
 
    # Stores length of longest subarray
    # having bitwise XOR equal to K
    maxLen = 0
 
    # Stores index of prefix
    # XOR of the array
    mp = {}
 
    # Insert 0 into the map
    mp[0] = -1
 
    # Traverse the array
    for i in range(N):
         
        # Update prefixXOR
        prefixXOR ^= arr[i]
 
        # If (prefixXOR ^ K) present
        # in the map
        if (prefixXOR ^ K) in mp:
             
            # Update maxLen
            maxLen = max(maxLen,
                        (i - mp[prefixXOR ^ K]))
 
        # If prefixXOR not present
        # in the Map
        else:
             
            # Insert prefixXOR
            # into the map
            mp[prefixXOR] = i
       
    return maxLen
 
# Driver Code
if __name__ == "__main__" :
 
    arr = [ 1, 2, 4, 7, 2 ]
    N = len(arr)
    K = 1
     
    print(LongestLenXORK(arr, N, K))
 
# This code is contributed by AnkThon

C#




// C# program to implement
// the above approach
using System;
using System.Collections.Generic;
class GFG
{
 
// Function to find the length of the longest
// subarray whose bitwise XOR is equal to K
static int longestLenXORK(int []arr,
                          int N, int K)
{
     
    // Stores prefix XOR
    // of the array
    int prefixXOR = 0;
 
    // Stores length of longest subarray
    // having bitwise XOR equal to K
    int maxLen = 0;
 
    // Stores index of prefix
    // XOR of the array
    Dictionary<int,
            int> mp = new Dictionary<int,
                                      int>();
                                       
    // Insert 0 into the map
    mp.Add(0, -1);
 
    // Traverse the array
    for(int i = 0; i < N; i++)
    {
         
        // Update prefixXOR
        prefixXOR ^= arr[i];
 
        // If (prefixXOR ^ K) present
        // in the map
        if (mp.ContainsKey(prefixXOR ^ K))
        {
             
            // Update maxLen
            maxLen = Math.Max(maxLen,
               (i - mp[prefixXOR ^ K]));
        }
         
        // If prefixXOR not present
        // in the Map
        if (!mp.ContainsKey(prefixXOR))
        {
             
            // Insert prefixXOR
            // into the map
            mp.Add(prefixXOR, i);
        }
    }
    return maxLen;
}
 
// Driver Code
public static void Main(String[] args)
{
    int []arr = {1, 2, 4, 7, 2};
    int N = arr.Length;
    int K = 1;
     
    Console.Write(longestLenXORK(arr, N, K));
}
}
 
// This code is contributed by shikhasingrajput

Output:

3

Time Complexity: O(N)
Auxiliary Space: O(N)

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