need improve204. Count Primes

int countPrimes(int n) {
if(--n < 2) return 0;
int m = (n + 1)/2, count = m, k, u = (sqrt(n) - 1)/2;
bool notPrime[m] = {0};

for(int i = 1; i <= u;i++)
if(!notPrime[i])
for(k = (i+ 1)*2*i; k < m;k += i*2 + 1)
if (!notPrime[k])
{
notPrime[k] = true;
count--;
}
return count;
}

class Solution {
public:
int countPrimes(int n) {
int *d=new int[n+1];
memset(d,0,(n+1)*sizeof(int));
int sum=0;
for(int i=2;i<n;i++)
{
if(d[i]==0)
{
sum++;
int index=i;
while(index<n)
{
d[index]=1;
index+=i;
}
}
}
return sum;
}
};

class Solution {
public:
int countPrimes(int n) {
if(n<3) return 0;
int *d=new int[n+1];
memset(d,0,(n+1)*sizeof(int));
int sum=1;
for(int i=3;i<n;i+=2)
{
if(d[i]==0)
{
sum++;
int index=i;
while(index<n)
{
d[index]=1;
index+=i;
}
}
}
return sum;
}
};