LeetCode204. Count Primes

一、题目

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

二、题解

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