Count of perfect squares of given length

Given an integer N, the task is to find the number of perfect squares of length N.
Examples:

Input: N = 1 
Output: 3 
Explanation: The single digit perfect squares are 1, 4 and 9.

Input: N = 2 
Output: 6 
Explanation: The two-digit perfect squares are 16, 25, 36, 49, 64 and 81. 

Naive Approach: In order to solve this problem, we can check all the numbers between 10^{( N – 1 )} and 10^N – 1 and increment the counter whenever we encounter a perfect square.
Below is the implementation of the above approach: 

C++

// C++ Program to count perfect
// squares of given length

#include <bits/stdc++.h>
using namespace std;

// Function to check if a
// number is perfect square
bool isPerfectSquare(long double x)
{
    // Find floating point value of
    // square root of x.
    long double sr = sqrt(x);

    // If square root is an integer
    return ((sr - floor(sr)) == 0);
}

// Function to return the count of
// n digit perfect squares
int countSquares(int n)
{
    // Initialize result
    int cnt = 0;

    // Traverse through all numbers
    // of n digits
    for (int i = pow(10, (n - 1));
         i < pow(10, n); i++) {

        // Check if current number
        // 'i' is perfect square
        if (i != 0 && isPerfectSquare(i))
            cnt++;
    }
    return cnt;
}

// Driver code
int main()
{
    int n = 3;
    cout << countSquares(n);
    return 0;
}

Java

// Java Program to count perfect
// squares of given length
import java.io.*;
import java.util.*;
class GFG{

// Function to check if a
// number is perfect square
static boolean isPerfectSquare(double x)
{

    // Find floating point value of
    // square root of x.
    double sr = Math.sqrt(x);

    // If square root is an integer
    return ((sr - Math.floor(sr)) == 0);
}

// Function to return the count of
// n digit perfect squares
static int countSquares(int n)
{

    // Initialize result
    int cnt = 0;

    // Traverse through all numbers
    // of n digits
    for(int i = (int) Math.pow(10, (n - 1));
                  i < Math.pow(10, n); i++)
    {

        // Check if current number
        // 'i' is perfect square
        if (i != 0 && isPerfectSquare(i))
            cnt++;
    }
    return cnt;
}

// Driver code
public static void main(String[] args)
{
    int n = 3;
    System.out.print(countSquares(n));
}
}

// This code is contributed by Rajput-Ji

Python3

# Python3 Program to count perfect
# squares of given length
import math;

# Function to check if a
# number is perfect square
def isPerfectSquare(x):

    # Find floating point value of
    # square root of x.
    sr = math.sqrt(x);

    # If square root is an integer
    return ((sr - math.floor(sr)) == 0);

# Function to return the count of
# n digit perfect squares
def countSquares(n):

    # Initialize result
    cnt = 0;

    # Traverse through all numbers
    # of n digits
    for i in range(int(math.pow(10, (n - 1))),
                   int(math.pow(10, n))):

        # Check if current number
        # 'i' is perfect square
        if (i != 0 and isPerfectSquare(i)):
            cnt += 1;

    return cnt;

# Driver code
n = 3;
print(countSquares(n));

# This code is contributed by Akanksha_Rai

C#

// C# program to count perfect
// squares of given length
using System;

class GFG{

// Function to check if a
// number is perfect square
static bool isPerfectSquare(double x)
{

    // Find floating point value of
    // square root of x.
    double sr = Math.Sqrt(x);

    // If square root is an integer
    return ((sr - Math.Floor(sr)) == 0);
}

// Function to return the count of
// n digit perfect squares
static int countSquares(int n)
{

    // Initialize result
    int cnt = 0;

    // Traverse through all numbers
    // of n digits
    for(int i = (int) Math.Pow(10, (n - 1));
                  i < Math.Pow(10, n); i++)
    {

       // Check if current number
       // 'i' is perfect square
       if (i != 0 && isPerfectSquare(i))
           cnt++;
    }
    return cnt;
}

// Driver code
public static void Main(String[] args)
{
    int n = 3;

    Console.Write(countSquares(n));
}
}

// This code is contributed by sapnasingh4991

Javascript

<script>
// Javascript Program to count perfect
// squares of given length

// Function to check if a
// number is perfect square
function isPerfectSquare(x)
{
    // Find floating point value of
    // square root of x.
    let sr = Math.sqrt(x);

    // If square root is an integer
    return ((sr - Math.floor(sr)) == 0);
}

// Function to return the count of
// n digit perfect squares
function countSquares(n)
{
    // Initialize result
    let cnt = 0;

    // Traverse through all numbers
    // of n digits
    for (let i = Math.pow(10, (n - 1));
         i < Math.pow(10, n); i++) {

        // Check if current number
        // 'i' is perfect square
        if (i != 0 && isPerfectSquare(i))
            cnt++;
    }
    return cnt;
}

// Driver code
let n = 3;
document.write(countSquares(n));

// This code is contributed by subhammahato348.
</script>

Output:

22

Time Complexity: O(10ⁿ*log(n))
Auxiliary Space: O(1)

Efficient Approach: In order to solve this problem we can simply find the number of perfect squares of length N using a formula:

1. A number n has d digits if 
2. Hence, a perfect square n² has d digits if or 
   
3. Thus, the required answer for count of perfect squares of N digits is 
   

Below is the implementation of the above approach:

C++

// C++ Program to count
// perfect squares of given length

#include <bits/stdc++.h>
using namespace std;

// Function to return the count of
// n digit perfect squares
int countSquares(int n)
{
    int r = ceil(sqrt(pow(10, n)));
    int l = ceil(sqrt(pow(10, n - 1)));
    return r - l;
}

// Driver code
int main()
{
    int n = 3;
    cout << countSquares(n);
    return 0;
}

Java

// Java Program to count perfect
// squares of given length
import java.io.*;
import java.util.*;
class GFG{

// Function to return the count
// of n digit perfect squares
static int countSquares(int n)
{
    int r = (int) Math.ceil(Math.sqrt
                           (Math.pow(10, n)));
    int l = (int) Math.ceil(Math.sqrt
                           (Math.pow(10, n - 1)));
    return r - l;
}

// Driver code
public static void main(String[] args)
{
    int n = 3;
    System.out.print(countSquares(n));
}
}

// This code is contributed by Rohit_ranjan

Python3

# Python3 program to count perfect
# squares of given length
import math

# Function to return the count
# of n digit perfect squares
def countSquares(n):

    r = math.ceil(math.sqrt
                 (math.pow(10, n)));
    l = math.ceil(math.sqrt
                 (math.pow(10, n - 1)));

    return r - l;

# Driver code
n = 3;
print(countSquares(n));

# This code is contributed by grand_master

C#

// C# Program to count perfect
// squares of given length
using System;
class GFG{

// Function to return the count
// of n digit perfect squares
static int countSquares(int n)
{
    int r = (int) Math.Ceiling(Math.Sqrt
                              (Math.Pow(10, n)));
    int l = (int) Math.Ceiling(Math.Sqrt
                              (Math.Pow(10, n - 1)));
    return r - l;
}

// Driver code
public static void Main()
{
    int n = 3;
    Console.Write(countSquares(n));
}
}

// This code is contributed by Nidhi_Biet

Javascript

<script>

// JavaScript Program to count
// perfect squares of given length

// Function to return the count of
// n digit perfect squares
function countSquares(n)
{
    let r = Math.ceil(Math.sqrt(Math.pow(10, n)));
    let l = Math.ceil(Math.sqrt(Math.pow(10, n - 1)));
    return r - l;
}

// Driver code
let n = 3;
document.write(countSquares(n));

</script>

Output:

22

Time Complexity: O(log N)
Auxiliary Space: O(1)