Description:

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

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public class Solution {
    public int countPrimes(int n) {
        boolean[] notPrime = new boolean[n];
        for (int i = 2; i * i < n; i++) {
            if (notPrime[i] == false) {
                for (int j = i * i; j < n; j += i) {
                    notPrime[j] = true;
                }
            }
        }
        int count = 0;
        for (int i = 2; i < n; i++) {
            if (notPrime[i] == false) {
                count++;
            }
        }
        return count;
    }
}