Find the number of zeroes (original) (raw)

Last Updated : 23 Jul, 2025

Given an array of 1s and 0s which has all 1s first followed by all 0s. Find the number of 0s. Count the number of zeroes in the given array.
Examples :

Input: arr[] = {1, 1, 1, 1, 0, 0} Output: 2

Input: arr[] = {1, 0, 0, 0, 0} Output: 4

Input: arr[] = {0, 0, 0} Output: 3

Input: arr[] = {1, 1, 1, 1} Output: 0

Approach 1: A simple solution is to traverse the input array. As soon as we find a 0, we return n - index of first 0. Here n is number of elements in input array. Time complexity of this solution would be O(n).

Implementation of above approach is below:

Try It Yourself

C++ `

// A program to find the number of zeros #include <bits/stdc++.h> using namespace std; int firstzeroindex(int arr[], int n) { for (int i = 0; i < n; i++) { if (arr[i] == 0) { return i; } } return -1; } int main() { int arr[] = { 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0 }; int n = sizeof(arr) / sizeof(arr[0]); int x = firstzeroindex(arr, n); if (x == -1) { cout << "Count of zero is 0" << endl; } else { cout << "count of zero is " << n - x << endl; } return 0; } // this code is contributed by machhaliya muhammad

Java

// A program to find the number of zeros

import java.io.*;

class GFG { static int firstzeroindex(int arr[], int n) { for (int i = 0; i < n; i++) { if (arr[i] == 0) { return i; } } return -1; } public static void main(String[] args) { int arr[] = { 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0 }; int n = arr.length; int x = firstzeroindex(arr, n); if (x == -1) { System.out.println("Count of zero is 0"); } else { System.out.print("count of zero is "); System.out.println(n-x); } } }

// This code is contributed by Abhijeet Kumar(abhijeet19403)

Python3

class GFG : @staticmethod def firstzeroindex( arr, n) : i = 0 while (i < n) : if (arr[i] == 0) : return i i += 1 return -1 @staticmethod def main( args) : arr = [1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0] n = len(arr) x = GFG.firstzeroindex(arr, n) if (x == -1) : print("Count of zero is 0") else : print("count of zero is ", end ="") print(n - x)

if name=="main": GFG.main([])

# This code is contributed by aadityaburujwale.

C#

// A program to find the number of zeros using System;

public class GFG{

      static int firstzeroindex(int[] arr, int n)
{
    for (int i = 0; i < n; i++) {
        if (arr[i] == 0) {
            return i;
        }
    }
    return -1;
}
public static void Main()
{
    int[] arr = { 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0 };
    int n = arr.Length;
    int x = firstzeroindex(arr, n);
    if (x == -1) {
          Console.WriteLine("Count of zero is 0");
    }
    else {
        Console.Write("count of zero is ");
          Console.WriteLine(n-x);
    }
}

}

// This code is contributed by Abhijeet Kumar(abhijeet19403)

JavaScript

Time complexity: O(n) where n is size of arr.

Space Complexity: O(1) as we are not using any extra space.

Approach 2: Since the input array is sorted, we can use Binary Search to find the first occurrence of 0. Once we have index of first element, we can return count as n - index of first zero.

Implementation:

C `

// A divide and conquer solution to find count of zeroes in an array // where all 1s are present before all 0s #include <stdio.h>

/* if 0 is present in arr[] then returns the index of FIRST occurrence of 0 in arr[low..high], otherwise returns -1 */ int firstZero(int arr[], int low, int high) { if (high >= low) { // Check if mid element is first 0 int mid = low + (high - low)/2; if (( mid == 0 || arr[mid-1] == 1) && arr[mid] == 0) return mid;

    if (arr[mid] == 1) // If mid element is not 0 
        return firstZero(arr, (mid + 1), high); 
    else // If mid element is 0, but not first 0 
        return firstZero(arr, low, (mid -1)); 
} 
return -1;

}

// A wrapper over recursive function firstZero() int countZeroes(int arr[], int n) { // Find index of first zero in given array int first = firstZero(arr, 0, n-1);

// If 0 is not present at all, return 0 
if (first == -1) 
    return 0; 

return (n - first);

}

/* Driver program to check above functions */ int main() { int arr[] = {1, 1, 1, 0, 0, 0, 0, 0}; int n = sizeof(arr)/sizeof(arr[0]); printf("Count of zeroes is %d", countZeroes(arr, n)); return 0; }

C++

// A divide and conquer solution to // find count of zeroes in an array // where all 1s are present before all 0s #include <bits/stdc++.h> using namespace std;

/* if 0 is present in arr[] then returns the index of FIRST occurrence of 0 in arr[low..high], otherwise returns -1 */ int firstZero(int arr[], int low, int high) { if (high >= low) { // Check if mid element is first 0 int mid = low + (high - low) / 2; if ((mid == 0 || arr[mid - 1] == 1) && arr[mid] == 0) return mid;

    // If mid element is not 0
    if (arr[mid] == 1) 
        return firstZero(arr, (mid + 1), high);
    
    // If mid element is 0, but not first 0    
    else 
        return firstZero(arr, low, (mid -1));
}
return -1;

}

// A wrapper over recursive function firstZero() int countZeroes(int arr[], int n) { // Find index of first zero in given array int first = firstZero(arr, 0, n - 1);

// If 0 is not present at all, return 0
if (first == -1)
    return 0;

return (n - first);

}

// Driver Code int main() { int arr[] = {1, 1, 1, 0, 0, 0, 0, 0}; int n = sizeof(arr) / sizeof(arr[0]); cout << "Count of zeroes is " << countZeroes(arr, n); return 0; }

// This code is contributed by SoumikMondal

Java

// A divide and conquer solution to find count of zeroes in an array // where all 1s are present before all 0s

class CountZeros { /* if 0 is present in arr[] then returns the index of FIRST occurrence of 0 in arr[low..high], otherwise returns -1 */ int firstZero(int arr[], int low, int high) { if (high >= low) { // Check if mid element is first 0 int mid = low + (high - low) / 2; if ((mid == 0 || arr[mid - 1] == 1) && arr[mid] == 0) return mid;

        if (arr[mid] == 1) // If mid element is not 0
            return firstZero(arr, (mid + 1), high);
        else // If mid element is 0, but not first 0
            return firstZero(arr, low, (mid - 1));
    }
    return -1;
}

// A wrapper over recursive function firstZero()
int countZeroes(int arr[], int n) 
{
    // Find index of first zero in given array
    int first = firstZero(arr, 0, n - 1);

    // If 0 is not present at all, return 0
    if (first == -1)
        return 0;

    return (n - first);
}

// Driver program to test above functions
public static void main(String[] args) 
{
    CountZeros count = new CountZeros();
    int arr[] = {1, 1, 1, 0, 0, 0, 0, 0};
    int n = arr.length;
    System.out.println("Count of zeroes is " + count.countZeroes(arr, n));
}

}

Python3

A divide and conquer solution to

find count of zeroes in an array

where all 1s are present before all 0s

if 0 is present in arr[] then returns

the index of FIRST occurrence of 0 in

arr[low..high], otherwise returns -1

def firstZero(arr, low, high):

if (high >= low):

    # Check if mid element is first 0
    mid = low + int((high - low) / 2)
    if (( mid == 0 or arr[mid-1] == 1)
                  and arr[mid] == 0):
        return mid
    
    # If mid element is not 0
    if (arr[mid] == 1): 
        return firstZero(arr, (mid + 1), high)
        
    # If mid element is 0, but not first 0
    else: 
        return firstZero(arr, low, (mid - 1))

return -1

A wrapper over recursive

function firstZero()

def countZeroes(arr, n):

# Find index of first zero in given array
first = firstZero(arr, 0, n - 1)

# If 0 is not present at all, return 0
if (first == -1):
    return 0

return (n - first)

Driver Code

arr = [1, 1, 1, 0, 0, 0, 0, 0] n = len(arr) print("Count of zeroes is", countZeroes(arr, n))

This code is contributed by Smitha Dinesh Semwal

C#

// A divide and conquer solution to find // count of zeroes in an array where all // 1s are present before all 0s using System;

class CountZeros { /* if 0 is present in arr[] then returns the index of FIRST occurrence of 0 in arr[low..high], otherwise returns -1 */ int firstZero(int []arr, int low, int high) { if (high >= low) { // Check if mid element is first 0 int mid = low + (high - low) / 2; if ((mid == 0 || arr[mid - 1] == 1) && arr[mid] == 0) return mid;

        if (arr[mid] == 1) // If mid element is not 0
            return firstZero(arr, (mid + 1), high);
            
        else // If mid element is 0, but not first 0
            return firstZero(arr, low, (mid - 1));
    }
    return -1;
}

// A wrapper over recursive function firstZero()
int countZeroes(int []arr, int n) 
{
    // Find index of first zero in given array
    int first = firstZero(arr, 0, n - 1);

    // If 0 is not present at all, return 0
    if (first == -1)
        return 0;

    return (n - first);
}

// Driver program to test above functions
public static void Main() 
{
    CountZeros count = new CountZeros();
    int []arr = {1, 1, 1, 0, 0, 0, 0, 0};
    int n = arr.Length;
    Console.Write("Count of zeroes is " + 
                   count.countZeroes(arr, n));
}

}

// This code is contributed by nitin mittal.

PHP

low,low, low,high) { if ($high >= $low) { // Check if mid element is first 0 mid=mid = mid=low + floor(($high - $low)/2); if (( mid==0∣∣mid == 0 || mid==0∣∣arr[$mid-1] == 1) && arr[arr[arr[mid] == 0) return $mid; // If mid element is not 0 if ($arr[$mid] == 1) return firstZero($arr, ($mid + 1), $high); // If mid element is 0, // but not first 0 else return firstZero($arr, $low, ($mid - 1)); } return -1; } // A wrapper over recursive // function firstZero() function countZeroes($arr, $n) { // Find index of first // zero in given array first=firstZero(first = firstZero(first=firstZero(arr, 0, $n - 1); // If 0 is not present // at all, return 0 if ($first == -1) return 0; return ($n - $first); } // Driver Code $arr = array(1, 1, 1, 0, 0, 0, 0, 0); n=sizeof(n = sizeof(n=sizeof(arr); echo("Count of zeroes is "); echo(countZeroes($arr, $n)); // This code is contributed by nitin mittal ?>

JavaScript

Output

Count of zeroes is 5

Time Complexity: O(Logn) where n is number of elements in arr[].

Auxiliary Space: O(logn)