CountDiv.java

package CountDiv;

class Solution {
    public int solution(int A, int B, int K) {
        // write your code in Java SE 8
        
        // need to achieve low complexity O(1)
        // using math equation (low complexity)
        
        // number of divisible values smaller than B
        int num_B = (int) Math.floor( B/K );
        // note: take "Math.floor" which is the basic number
        
        // number of divisible values smaller than A
        int num_A = (int) Math.floor( A/K );
        // note: take "Math.floor" which is the basic number
        
        // number of divisible numbers
        int num_div = num_B - num_A;
        
        // note: plus one (if A % K == 0)
        // because "A" is also divisble 
        // without "plus", "A" will be deducted
        int plus = 0;
        if(A % K == 0)
            plus = 1;
            
        // num_div + plus
        num_div = num_div + plus;
        
        // return the number of K-divisible values between A and B
        return num_div;
    }
}