# Number of ways to erase exactly one element in the Binary Array to make XOR zero

• Last Updated : 19 Mar, 2022

Given a binary array of 0’s and 1’s, the task is to find the number of ways to erase exactly one element from this array to make XOR zero.
Examples:

```Input: arr = {1, 1, 1, 1, 1 }
Output: 5
You can erase any of the given 5 1's,
it will make the XOR of the rest equal to zero.

Input: arr = {1, 0, 0, 1, 0 }
Output: 3
Since the XOR of array is already 0,
You can erase any of the given 3 0's
so that the XOR remains unaffected.```

Approach: Since we know that, to make XOR of binary elements be 0, the number of 1’s should be even. Hence this problem can be broken into 4 cases:

• When the number of 1’s is even and the number of 0’s is also even in the given array: In this scenario, the XOR of the array is already 0. Hence to keep the XOR unaffected, we can remove only the 0’s. Hence the number of ways to erase exactly one element from this array to make XOR zero is the number of 0’s in this array.

• When the number of 1’s is even and the number of 0’s is odd in the given array: In this scenario, the XOR of the array is already 0. Hence to keep the XOR unaffected, we can remove only the 0’s. Hence the number of ways to erase exactly one element from this array to make XOR zero is the number of 0’s in this array.

• When the number of 1’s is odd and the number of 0’s is even in the given array: In this scenario, the XOR of the array is 1. Hence to make the XOR 0, we can remove any of the 1’s. Hence the number of ways to erase exactly one element from this array to make XOR zero is the number of 1’s in this array.

• When the number of 1’s is odd and the number of 0’s is also odd in the given array: In this scenario, the XOR of the array is 1. Hence to make the XOR 0, we can remove any of the 1’s. Hence the number of ways to erase exactly one element from this array to make XOR zero is the number of 1’s in this array.

Below is the implementation of the above approach:

## C++

 `// C++ program to find the number of ways``// to erase exactly one element``// from this array to make XOR zero` `#include ``using` `namespace` `std;` `// Function to find the number of ways``int` `no_of_ways(``int` `a[], ``int` `n)``{``    ``int` `count_0 = 0, count_1 = 0;` `    ``// Calculate the number of 1's and 0's``    ``for` `(``int` `i = 0; i < n; i++) {``        ``if` `(a[i] == 0)``            ``count_0++;``        ``else``            ``count_1++;``    ``}` `    ``// Considering the 4 cases``    ``if` `(count_1 % 2 == 0)``        ``return` `count_0;``    ``else``        ``return` `count_1;``}` `// Driver code``int` `main()``{``    ``int` `n = 4;``    ``int` `a1 = { 1, 1, 0, 0 };``    ``cout << no_of_ways(a1, n) << endl;` `    ``n = 5;``    ``int` `a2 = { 1, 1, 1, 0, 0 };``    ``cout << no_of_ways(a2, n) << endl;` `    ``n = 5;``    ``int` `a3 = { 1, 1, 0, 0, 0 };``    ``cout << no_of_ways(a3, n) << endl;` `    ``n = 6;``    ``int` `a4 = { 1, 1, 1, 0, 0, 0 };``    ``cout << no_of_ways(a4, n) << endl;` `    ``return` `0;``}`

## Java

 `// Java program to find the number of ways``// to erase exactly one element``// from this array to make XOR zero``class` `GFG``{``    ` `    ``// Function to find the number of ways``    ``static` `int` `no_of_ways(``int` `a[], ``int` `n)``    ``{``        ``int` `count_0 = ``0``, count_1 = ``0``;``    ` `        ``// Calculate the number of 1's and 0's``        ``for` `(``int` `i = ``0``; i < n; i++)``        ``{``            ``if` `(a[i] == ``0``)``                ``count_0++;``            ``else``                ``count_1++;``        ``}``    ` `        ``// Considering the 4 cases``        ``if` `(count_1 % ``2` `== ``0``)``            ``return` `count_0;``        ``else``            ``return` `count_1;``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `main (String[] args)``    ``{``        ``int` `n = ``4``;``        ``int` `a1[] = { ``1``, ``1``, ``0``, ``0` `};``        ``System.out.println(no_of_ways(a1, n));``    ` `        ``n = ``5``;``        ``int` `a2[] = { ``1``, ``1``, ``1``, ``0``, ``0` `};``        ``System.out.println(no_of_ways(a2, n));``    ` `        ``n = ``5``;``        ``int` `a3[] = { ``1``, ``1``, ``0``, ``0``, ``0` `};``        ``System.out.println(no_of_ways(a3, n));``    ` `        ``n = ``6``;``        ``int` `a4[] = { ``1``, ``1``, ``1``, ``0``, ``0``, ``0` `};``        ``System.out.println(no_of_ways(a4, n));``    ``}``}` `// This code is contributed by AnkitRai01`

## Python3

 `# Python3 program to find the number of ways``# to erase exactly one element``# from this array to make XOR zero` `# Function to find the number of ways``def` `no_of_ways(a, n):` `    ``count_0 ``=` `0``    ``count_1 ``=` `0` `    ``# Calculate the number of 1's and 0's``    ``for` `i ``in` `range``(``0``, n):``        ``if` `(a[i] ``=``=` `0``):``            ``count_0 ``+``=` `1``        ``else``:``            ``count_1 ``+``=` `1``    ` `    ``# Considering the 4 cases``    ``if` `(count_1 ``%` `2` `=``=` `0``):``        ``return` `count_0``    ``else``:``        ``return` `count_1` `# Driver code``if` `__name__ ``=``=` `'__main__'``:``    ``n ``=` `4``    ``a1 ``=` `[ ``1``, ``1``, ``0``, ``0` `]``    ``print``(no_of_ways(a1, n))` `    ``n ``=` `5``    ``a2 ``=` `[ ``1``, ``1``, ``1``, ``0``, ``0` `]``    ``print``(no_of_ways(a2, n))` `    ``n ``=` `5``    ``a3 ``=` `[ ``1``, ``1``, ``0``, ``0``, ``0` `]``    ``print``(no_of_ways(a3, n))` `    ``n ``=` `6``    ``a4 ``=` `[ ``1``, ``1``, ``1``, ``0``, ``0``, ``0` `]``    ``print``(no_of_ways(a4, n))` `# This code is contributed by ashutosh450`

## C#

 `// C# program to find the number of ways``// to erase exactly one element``// from this array to make XOR zero``using` `System;` `class` `GFG``{``    ` `    ``// Function to find the number of ways``    ``static` `int` `no_of_ways(``int` `[]a, ``int` `n)``    ``{``        ``int` `count_0 = 0, count_1 = 0;``    ` `        ``// Calculate the number of 1's and 0's``        ``for` `(``int` `i = 0; i < n; i++)``        ``{``            ``if` `(a[i] == 0)``                ``count_0++;``            ``else``                ``count_1++;``        ``}``    ` `        ``// Considering the 4 cases``        ``if` `(count_1 % 2 == 0)``            ``return` `count_0;``        ``else``            ``return` `count_1;``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `Main ()``    ``{``        ``int` `n = 4;``        ``int` `[] a1 = { 1, 1, 0, 0 };``        ``Console.WriteLine(no_of_ways(a1, n));``    ` `        ``n = 5;``        ``int` `[] a2 = { 1, 1, 1, 0, 0 };``        ``Console.WriteLine(no_of_ways(a2, n));``    ` `        ``n = 5;``        ``int` `[] a3 = { 1, 1, 0, 0, 0 };``        ``Console.WriteLine(no_of_ways(a3, n));``    ` `        ``n = 6;``        ``int` `[] a4 = { 1, 1, 1, 0, 0, 0 };``        ``Console.WriteLine(no_of_ways(a4, n));``    ``}``}` `// This code is contributed by Mohit kumar`

## Javascript

 ``

Output:

```2
3
3
3```

Time Complexity: O(n)

Auxiliary Space: O(1)

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