Count the Number of Ideal Arrays
<p>You are given two integers <code>n</code> and <code>maxValue</code>, which are used to describe an <strong>ideal</strong> array.</p>

<p>A <strong>0-indexed</strong> integer array <code>arr</code> of length <code>n</code> is considered <strong>ideal</strong> if the following conditions hold:</p>

<ul>
	<li>Every <code>arr[i]</code> is a value from <code>1</code> to <code>maxValue</code>, for <code>0 &lt;= i &lt; n</code>.</li>
	<li>Every <code>arr[i]</code> is divisible by <code>arr[i - 1]</code>, for <code>0 &lt; i &lt; n</code>.</li>
</ul>

<p>Return <em>the number of <strong>distinct</strong> ideal arrays of length </em><code>n</code>. Since the answer may be very large, return it modulo <code>10<sup>9</sup> + 7</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 2, maxValue = 5
<strong>Output:</strong> 10
<strong>Explanation:</strong> The following are the possible ideal arrays:
- Arrays starting with the value 1 (5 arrays): [1,1], [1,2], [1,3], [1,4], [1,5]
- Arrays starting with the value 2 (2 arrays): [2,2], [2,4]
- Arrays starting with the value 3 (1 array): [3,3]
- Arrays starting with the value 4 (1 array): [4,4]
- Arrays starting with the value 5 (1 array): [5,5]
There are a total of 5 + 2 + 1 + 1 + 1 = 10 distinct ideal arrays.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 5, maxValue = 3
<strong>Output:</strong> 11
<strong>Explanation:</strong> The following are the possible ideal arrays:
- Arrays starting with the value 1 (9 arrays): 
   - With no other distinct values (1 array): [1,1,1,1,1] 
   - With 2<sup>nd</sup> distinct value 2 (4 arrays): [1,1,1,1,2], [1,1,1,2,2], [1,1,2,2,2], [1,2,2,2,2]
   - With 2<sup>nd</sup> distinct value 3 (4 arrays): [1,1,1,1,3], [1,1,1,3,3], [1,1,3,3,3], [1,3,3,3,3]
- Arrays starting with the value 2 (1 array): [2,2,2,2,2]
- Arrays starting with the value 3 (1 array): [3,3,3,3,3]
There are a total of 9 + 1 + 1 = 11 distinct ideal arrays.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>2 &lt;= n &lt;= 10<sup>4</sup></code></li>
	<li><code>1 &lt;= maxValue &lt;= 10<sup>4</sup></code></li>
</ul>
Math; Dynamic Programming; Combinatorics; Number Theory
class Solution {
    public int idealArrays(int n, int maxValue) {
        final int mod = (int) 1e9 + 7;
        int[][] c = new int[n][16];
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j <= i && j < 16; ++j) {
                c[i][j] = j == 0 ? 1 : (c[i - 1][j] + c[i - 1][j - 1]) % mod;
            }
        }
        long[][] f = new long[maxValue + 1][16];
        for (int i = 1; i <= maxValue; ++i) {
            f[i][1] = 1;
        }
        for (int j = 1; j < 15; ++j) {
            for (int i = 1; i <= maxValue; ++i) {
                int k = 2;
                for (; k * i <= maxValue; ++k) {
                    f[k * i][j + 1] = (f[k * i][j + 1] + f[i][j]) % mod;
                }
            }
        }
        long ans = 0;
        for (int i = 1; i <= maxValue; ++i) {
            for (int j = 1; j < 16; ++j) {
                ans = (ans + f[i][j] * c[n - 1][j - 1]) % mod;
            }
        }
        return (int) ans;
    }
}
