Question

204. Count Primes

中文文档

Description

Given an integer n, return _the number of prime numbers that are strictly less than_ n.

 

Example 1:

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

Example 2:

Input: n = 0
Output: 0

Example 3:

Input: n = 1
Output: 0

 

Constraints:

• 0 <= n <= 5 * 106

Solutions

Solution 1

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

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

class Solution {
public:
  int countPrimes(int n) {
    vector<bool> primes(n, true);
    int ans = 0;
    for (int i = 2; i < n; ++i) {
      if (primes[i]) {
        ++ans;
        for (int j = i; j < n; j += i) primes[j] = false;
      }
    }
    return ans;
  }
};

func countPrimes(n int) int {
  primes := make([]bool, n)
  for i := range primes {
    primes[i] = true
  }
  ans := 0
  for i := 2; i < n; i++ {
    if primes[i] {
      ans++
      for j := i + i; j < n; j += i {
        primes[j] = false
      }
    }
  }
  return ans
}

/**
 * @param {number} n
 * @return {number}
 */
var countPrimes = function (n) {
  let primes = new Array(n).fill(true);
  let ans = 0;
  for (let i = 2; i < n; ++i) {
    if (primes[i]) {
      ++ans;
      for (let j = i + i; j < n; j += i) {
        primes[j] = false;
      }
    }
  }
  return ans;
};

public class Solution {
  public int CountPrimes(int n) {
    var notPrimes = new bool[n];
    int ans = 0;
    for (int i = 2; i < n; ++i)
    {
      if (!notPrimes[i])
      {
        ++ans;
        for (int j = i + i; j < n; j += i)
        {
          notPrimes[j] = true;
        }
      }
    }
    return ans;
  }
}