This solution is OK. But it not satisfy the restrictions.
Your tests are too weak.

#include <iostream>
int main(int argc, char* argv[])
{
    int i= 0;
    for (i = 0; i < 1000000; i++)
            {
                printf("%c", rand()%26 + 'a');
            }
    return 0;
}

I think that you miss conditions:
Every possible subsequence with two letters length occurs not more than 2 000 times;
Every possible subsequence with three letters length occurs not more than 100 times;

I think with just a few tweaks, and "some" luck you can get AC on 1589.