Problem:
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.
思路：
Solution (C++):
int countPrimes(int n) {
int count = 0;
vector<bool> isPrime(n, false);
for (int i = 2; i < n; ++i) {
isPrime[i] = true;
}
for (int i = 2; i*i < n; ++i) {
if (!isPrime[i]) continue;
for (int j = i * i; j < n; j += i) {
isPrime[j] = false;
}
}
for (int i = 2; i < n; ++i) {
if (isPrime[i]) ++count;
}
return count;
}
性能：
Runtime: 208 ms Memory Usage: 6.6 MB
思路：
Solution (C++):
性能：
Runtime: ms Memory Usage: MB
