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