LeetCode/204. CountPrimes

Link

• https://leetcode.com/problems/count-primes/

Summary

Count primes. Was struggling thinking how to remove primes. Answer was written in wikipedia. https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes#Algorithm_complexity

Need to think how to translate human language to programming & think logically.

Solution

public class Solution {
    List<int> list = new List<int>();

    public int CountPrimes(int n)
    {
        bool[] isPrime = new bool[n];
        for (int i = 2; i < n; i++) {
            isPrime[i] = true;
        }
        // Loop's ending condition is i * i < n instead of i < sqrt(n)
        // to avoid repeatedly calling an expensive function sqrt().
        for (int i = 2; i * i < n; i++) {
            if (!isPrime[i]) continue;
            for (int j = i * i; j < n; j += i) {
                isPrime[j] = false;
            }
        }
        int count = 0;
        for (int i = 2; i < n; i++) {
            if (isPrime[i]) count++;
        }
        return count;
    }
}