[codility]CountDiv

https://codility.com/demo/take-sample-test/count_div

此题比较简单，是在O(1)时间里求区间[A,B]里面能被K整除的数字，那么就计算一下就能得到。

int solution(int A, int B, int K) {
    // write your code in C++11
    int diff = (B-A+1);
    int result = diff / K;
    for (int i = A + result * K; i <= B; i++)
    {
        if (i % K == 0)
            result++;
    }
    return result;
}