Title description
Count the number of all prime numbers less than a non-negative integer n.
Example
示例 1:
输入:n = 10
输出:4
解释:小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。
示例 2:
输入:n = 0
输出:0
示例 3:
输入:n = 1
输出:0
prompt
0 <= n <= 5 * 106
Problem solving ideas
First edition: simply count whether the number less than n is prime
Code
bool isPrime(int num){
for(int i=2;i*i<=num;i++){
if(num%i==0)return false;
}return true;
}
int countPrimes(int n){
int count=0;
if(n<=1)return 0;
for(int i=2;i<n;i++){
if(isPrime(i))count++;
}return count;
}