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:

- Every prime number other than 2 is an odd integer. So, after
**2,**we can check for odd integers only. - 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

- If the number is less than
**3**, return**0,**as 2 is the smallest prime - Run a loop checking all numbers, starting from 3
- A number, N is prime if:
- It has
**0**prime factors between 2 and**√N**

- It has
- If the number is prime, increment result
- Print the result

### 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 **0**

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.

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**.

### Algorithm

- Maintain a boolean array
**prime**of size equal to**N – 1** **prime[]**is used to mark composites and checking primes at the end- Initialise prime of every integer as
**true**. The algorithm sets**false**to every non-prime number - 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.

- For every integer x, if it has prime[x] as true, mark all its multiple as
- Finally check how many numbers in the range [2, N – 1] have
**true**in the prime table - 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:

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.