README_EN.md

204. Count Primes

中文文档

Description

Count the number of prime numbers less than a non-negative number, n.

Example:

Input: 10

Output: 4

Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

Solutions

Python3

class Solution:
    def countPrimes(self, n: int) -> int:
        if n < 2:
            return 0
        res = 0
        primes = [True for _ in range(n)]
        for i in range(2, n):
            if primes[i]:
                res += 1
                for j in range(i * i, n, i):
                    primes[j] = False
        return res

Java

class Solution {
    public int countPrimes(int n) {
        if (n < 2) return 0;
        boolean[] primes = new boolean[n];
        Arrays.fill(primes, true);
        int res = 0;
        for (int i = 2; i < n; ++i) {
            if (primes[i]) {
                ++res;
                if ((long) i * i < n) {
                    for (int j = i * i; j < n; j += i) {
                        primes[j] = false;
                    }
                }
            }
        }
        return res;
    }
}

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