Count Primes

Desicription

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

Example:

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

Solution

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