LeetCode 204. Count Primes

原题链接在这里：https://leetcode.com/problems/count-primes/

题目：

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

题解：

网上查查，原来有一种方法叫：Sieve of Eratosthenes 的方法.

Time Complexity: O(sqrt(n) * loglogn).

Space: O(n).

AC Java:

public class Solution {
    public int countPrimes(int n) {
        if(n < 3){
            return 0;
        }
        boolean [] isPrime = new boolean[n];
        Arrays.fill(isPrime, true);
        int res = n-2;
        for(int i = 2; i*i<n; i++){
            if(isPrime[i]){
                for(int j = i; i*j<n; j++){
                    if(isPrime[i*j]){
                        isPrime[i*j] = false;
                        res--;
                    }
                }
            }
        }
        return res;
    }
}

 跟上Perfect Squares