CountDiv.js

/*
CountDiv
Compute number of integers divisible by k in range [a..b].

Write a function:

function solution(A, B, K);

that, given three integers A, B and K, returns the number of integers within the range [A..B] that are divisible by K, i.e.:

{ i : A ≤ i ≤ B, i mod K = 0 }

For example, for A = 6, B = 11 and K = 2, your function should return 3, because there are three numbers divisible by 2 within the range [6..11], namely 6, 8 and 10.

Write an efficient algorithm for the following assumptions:

A and B are integers within the range [0..2,000,000,000];
K is an integer within the range [1..2,000,000,000];
A ≤ B.

*/

function solution(A, B, K) {
    // write your code in JavaScript (Node.js 8.9.4)
    var offset = A % K === 0 ? 1 : 0;
    return Math.floor(B / K) - Math.floor(A / K) + offset;
}