Count Primes Leetcode Solutions  

Difficulty Level Easy
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In this problem, we are given an integer, N. The goal is to count how numbers less than N, are primes. The integer is constrained to be non-negative.

Example  

7
3
10
4

Explanation

Primes less than 10 are 2, 3, 5 and 7. So, the count is 4.

Approach(Brute Force)  

The general approach is to check for every integer less than N and increment the result if they are prime. For example, consider N = 10. Now, we can run a check from 2 to N – 1 to find how many primes lie in this range. But, this approach requires a prime check on the whole range, [2, N – 1].  Therefore, this is slow.

We can do some optimizations like:

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  1. Every prime number other than 2 is an odd integer. So, after 2, we can check for odd integers only.
  2. We can check if a number is prime in O( √N) time to improve the algorithm.

However, we still have a better approach, discussed later in this article.

Algorithm

  1. If the number is less than 3, return 0, as 2 is the smallest prime
  2. Run a loop checking all numbers, starting from 3
  3. A number, N is prime if:
    • It has prime factors between 2 and √N
  4. If the number is prime, increment result
  5. Print the result
See also
Find First and Last Position of Element in Sorted Array Leetcode Solution

Implementation of Count Primes Leetcode Solutions

C++ Program

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

bool isPrime(int N)
{
    for(int i = 2 ; i * i <= N ; i++)
        if(N % i == 0)
            return false;
    return true;
}


int countPrimes(int N)
{
    if(N < 3)
        return 0;
    int cnt = 1;
    for(int i = 3 ; i < N ; i += 2)
        if(isPrime(i))
            cnt++;

    return cnt;
}


int main()
{
    int N = 10;
    cout << countPrimes(N) << '\n';
}

Java Program

class count_primes
{
    static boolean isPrime(int N)
    {
        for(int i = 2 ; i * i <= N ; i++)
            if(N % i == 0)
                return false;

        return true;
    }

    static int countPrimes(int N)
    {
        if(N < 3)
            return 0;
        int cnt = 1;//since number is atleast 3, 2 is a prime less than N
        for(int i = 3 ; i < N ; i += 2)
            if(isPrime(i))
                cnt++;
        return cnt;
    }

    public static void main(String args[])
    {
        int N = 10;
        System.out.println(countPrimes(N));
    }
}

4

Complexity Analysis of Count Primes Leetcode Solutions

Time Complexity

We run a loop for N/2 times. In every check, a time of complexity O(N / 2) (taking average as N / 2) is spent. So, the time complexity of this algorithm is O(N√N).

Space complexity

O(1), Only constant space is used for constant variables.

Approach(Optimal Method)  

Consider any prime number, let say 5, then it is sure that all the multiples of 5, other than itself can never be prime as they would have 5 as one of their prime factors. Similarly, we can store all the numbers less than N and greater than

The Sieve of Eratosthenes is an ancient and efficient algorithm to find primes less than N when N is small enough to allocate O(N) space in the memory. It iteratively eliminates all the non-prime numbers by marking them as composite. After all the composites are marked, the numbers that are unmarked are primes.

See also
Roman to Integer Leetcode Solution

Also, we need to store a boolean array to check if any number is marked or not, which accounts for memory usage. So, this is a case where we trade memory off to improve on time.

Count Primes Leetcode SolutionsPin

Algorithm

  1. Maintain a boolean array prime of size equal to N – 1
  2. prime[] is used to mark composites and checking primes at the end
  3. Initialise prime of every integer as true. The algorithm sets false to every non-prime number
  4. Run a loop of consecutive integers, starting from 2 until the first multiple of the integer is less than N:
    • For every integer x, if it has prime[x] as true, mark all its multiple as false
    • The multiple of every integer here starts from x * x as all other multiples of x will already be unmarked.
  5. Finally check how many numbers in the range [2, N – 1] have true in the prime table
  6. Print the result

Implementation of Count Primes Leetcode Solutions

C++ Program

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

int sieve(int N)
{
    if(N < 3)
        return 0;

    int cnt = 0;
    bool prime[N];
    for(int i = 2 ; i < N ; i++)
        prime[i] = true;

    for(int i = 2 ; i * i < N ; i++)
    {
        if(prime[i])
            for(int j = i * i ; j < N ; j += i)
                prime[j] = false;
    }

    for(int i = 2 ; i < N ; i++)
        if(prime[i])
            cnt++;

    return cnt;
}

int countPrimes(int N)
{
    if(N < 3)
        return 0;
    return sieve(N);
}


int main()
{
    int N = 10;
    cout << countPrimes(N) << '\n';
}

Java Program

class count_primes
{
    static int sieve(int N)
    {
        if(N < 3)
            return 0;

        int cnt = 0;
        boolean[] prime= new boolean[N];
        for(int i = 2 ; i < N ; i++)
            prime[i] = true;

        for(int i = 2 ; i * i < N ; i++)
        {
            if(prime[i])
                for(int j = i * i ; j < N ; j += i)
                    prime[j] = false;
        }

        for(int i = 2 ; i < N ; i++)
            if(prime[i])
                cnt++;

        return cnt;
    }

    static int countPrimes(int N)
    {
        if(N < 3)
            return 0;
        return sieve(N);
    }

    public static void main(String args[])
    {
        int N = 10;
        System.out.println(countPrimes(N));
    }
}

4

Complexity Analysis of Count Primes Leetcode Solutions

Time Complexity

The complexity of the algorithm is O(Nlog(logN)). This can be proved as:

See also
Path Sum

For every integer, X, we spend a time of N / X eliminating all its multiples.

So, the time spent in total is: N/2 + N/3 + N/5 + N/7 + ……

Taking N as common, T(N) = N (1/2 + 1/3 + 1/5 + 1/7 + …). Sum of series (1/2 + 1/3 + 1/5 + 1/7 + …) can be proved as log(logN).

Therefore, T(N) = Nlog(logN)

Space complexity

O(N), as we create a hash table to store if a number is prime or not.

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