204. Count Primes
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:**这道题可以考虑用打表的方式来解，但考虑到n值会很大，用数组存储会爆的，所以考虑用指针来保存。
int countPrimes(int n) {
int *isPrime=(int *)malloc(n*sizeof(int));
memset(isPrime,0,sizeof(isPrime));
for(int i=2;i*i<n;i++)
{
if(isPrime[i]) continue;
for(int j=i*i;j<n;j+=i)
{
isPrime[j]=1;
}
}
int count=0;
for(int i=2;i<n;i++)
{
if(!isPrime[i]) count++;
}
return count;
}