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Longest subarray having difference in the count of 1’s and 0’s equal to k
  • Last Updated : 03 Sep, 2019

Given a binary array arr[] of size n and a value k. The task is to find the length of the longest subarray having difference in the count of 1’s and 0’s equal to k. The count of 1’s should be equal to or greater than the count of 0’s in the subarray according to the value of k.

Examples:

Input: arr[] = {0, 1, 1, 0, 1}, k = 2
Output: 4
The highlighted portion is the required subarray
{0, 1, 1, 0, 1}. In the subarray count of 1's is 3
and count of 0's is 1. 
Therefore, difference in count = 3 - 1 = 2.

Input: arr[] = {1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1}, k = 0
Output: 6
The highlighted portion is the required subarray
{1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1}. In the subarray 
count of 1's is 3 and count of 0's is 3. 
Therefore, difference in count = 3 - 3 = 0.

Naive Approach: Consider the difference in the count of 1’s and 0’s of all the sub-arrays and return the length of the longest sub-array having required difference equal to ‘k’. Time Complexity will be O(n^2).

Efficient Approach: This problem is a variation of finding the longest sub-array having sum k. Replace all the 0’s in the arr[] with -1 and then find the longest subarray of ‘arr’ having sum equal to ‘k’.

Below is the implementation of above approach:

C++






// C++ implementation of above approach
#include <bits/stdc++.h>
using namespace std;
  
// function to find the length of the longest
// subarray having difference in the count
// of 1's and 0's equal to k
int lenOfLongSubarr(int arr[], int n, int k)
{
  
    // unordered_map 'um' implemented
    // as hash table
    unordered_map<int, int> um;
    int sum = 0, maxLen = 0;
  
    // traverse the given array
    for (int i = 0; i < n; i++) {
  
        // accumulate sum
        sum += ((arr[i] == 0) ? -1 : arr[i]);
  
        // when subarray starts from index '0'
        if (sum == k)
            maxLen = i + 1;
  
        // make an entry for 'sum' if it is
        // not present in 'um'
        if (um.find(sum) == um.end())
            um[sum] = i;
  
        // check if 'sum-k' is present in 'um'
        // or not
        if (um.find(sum - k) != um.end()) {
  
            // update maxLength
            if (maxLen < (i - um[sum - k]))
                maxLen = i - um[sum - k];
        }
    }
  
    // required maximum length
    return maxLen;
}
  
// Driver Code
int main()
{
    int arr[] = { 0, 1, 1, 0, 1 };
    int n = sizeof(arr) / sizeof(arr[0]);
    int k = 2;
    cout << "Length = "
         << lenOfLongSubarr(arr, n, k);
  
    return 0;
}

Java




// Java implementation of the above approach.
import java.util.HashMap;
import java.util.Map;
  
class GfG
{
  
    // Function to find the length of the longest 
    // subarray having difference in the count 
    // of 1's and 0's equal to k 
    static int lenOfLongSubarr(int arr[], int n, int k) 
    
        // unordered_map 'um' implemented 
        // as hash table 
        HashMap<Integer, Integer> um = new HashMap<>(); 
        int sum = 0, maxLen = 0
      
        // traverse the given array 
        for (int i = 0; i < n; i++) 
        
      
            // accumulate sum 
            sum += ((arr[i] == 0) ? -1 : arr[i]); 
      
            // when subarray starts from index '0' 
            if (sum == k) 
                maxLen = i + 1
      
            // make an entry for 'sum' if 
            // it is not present in 'um' 
            if (!um.containsKey(sum)) 
                um.put(sum, i); 
      
            // check if 'sum-k' is present 
            // in 'um' or not 
            if (um.containsKey(sum - k)) 
            
      
                // update maxLength 
                if (maxLen < (i - um.get(sum - k))) 
                    maxLen = i - um.get(sum - k); 
            
        
      
        // required maximum length 
        return maxLen; 
    }
  
    // Driver code
    public static void main(String []args)
    {
          
        int arr[] = { 0, 1, 1, 0, 1 }; 
        int n = arr.length; 
        int k = 2
  
        System.out.println("Length = " + lenOfLongSubarr(arr, n, k));
    }
}
  
// This code is contributed by Rituraj Jain

Python




# Python3 implementation of above approach
  
# function to find the length of the longest
# subarray having difference in the count
# of 1's and 0's equal to k
def lenOfLongSubarr(arr, n, k):
  
    # unordered_map 'um' implemented
    # as hash table
    um = dict()
  
    Sum, maxLen = 0, 0
  
    # traverse the given array
    for i in range(n):
  
        # accumulate sum
        if arr[i] == 0:
            Sum += -1
        else:
            Sum+=arr[i]
  
        # when subarray starts from index '0'
        if (Sum == k):
            maxLen = i + 1
  
        # make an entry for 'Sum' if it is
        # not present in 'um'
        if (Sum not in um.keys()):
            um[Sum] = i
  
        # check if 'Sum-k' is present in 'um'
        # or not
        if ((Sum - k) in um.keys()):
  
            # update maxLength
            if (maxLen < (i - um[Sum - k])):
                maxLen = i - um[Sum - k]
  
    # required maximum length
    return maxLen
  
# Driver Code
arr = [0, 1, 1, 0, 1]
n = len(arr)
k = 2
print("Length = ",lenOfLongSubarr(arr, n, k))
  
# This code is contributed by mohit kumar

C#




// C# implementation of the approach
using System;
using System.Collections.Generic;
  
class GFG
{
  
    // Function to find the length of the longest 
    // subarray having difference in the count 
    // of 1's and 0's equal to k 
    static int lenOfLongSubarr(int []arr,
                               int n, int k) 
    
        // unordered_map 'um' implemented 
        // as hash table 
        Dictionary<int
                   int> um = new Dictionary<int
                                            int>();
        int sum = 0, maxLen = 0; 
      
        // traverse the given array 
        for (int i = 0; i < n; i++) 
        
      
            // accumulate sum 
            sum += ((arr[i] == 0) ? -1 : arr[i]); 
      
            // when subarray starts from index '0' 
            if (sum == k) 
                maxLen = i + 1; 
      
            // make an entry for 'sum' if 
            // it is not present in 'um' 
            if (!um.ContainsKey(sum)) 
                um.Add(sum, i); 
      
            // check if 'sum-k' is present 
            // in 'um' or not 
            if (um.ContainsKey(sum - k)) 
            
      
                // update maxLength 
                if (maxLen < (i - um[sum - k])) 
                    maxLen = i - um[sum - k]; 
            
        
      
        // required maximum length 
        return maxLen; 
    }
  
    // Driver code
    public static void Main(String []args)
    {
          
        int []arr = { 0, 1, 1, 0, 1 }; 
        int n = arr.Length; 
        int k = 2; 
  
        Console.WriteLine("Length = "
                lenOfLongSubarr(arr, n, k));
    }
}
  
// This code is contributed by Princi Singh
Output:
Length = 4


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

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