Count Prime
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.
solution 1:
class Solution {
public int countPrimes(int n) {
boolean[] prime = new boolean[n];
Arrays.fill(prime, true);
for(int i = 2; i < n; i++){
if(prime[i]){
// 将i的2倍、3倍、4倍...都标记为非素数
for(int j = i * 2; j < n; j = j + i){
prime[j] = false;
}
}
}
int count = 0;
for(int i = 2; i < n; i++){
if(prime[i]) count++;
}
return count;
}
}