Count Ways To Build Good Strings
<p>Given the integers <code>zero</code>, <code>one</code>, <code>low</code>, and <code>high</code>, we can construct a string by starting with an empty string, and then at each step perform either of the following:</p>

<ul>
	<li>Append the character <code>&#39;0&#39;</code> <code>zero</code> times.</li>
	<li>Append the character <code>&#39;1&#39;</code> <code>one</code> times.</li>
</ul>

<p>This can be performed any number of times.</p>

<p>A <strong>good</strong> string is a string constructed by the above process having a <strong>length</strong> between <code>low</code> and <code>high</code> (<strong>inclusive</strong>).</p>

<p>Return <em>the number of <strong>different</strong> good strings that can be constructed satisfying these properties.</em> Since the answer can be large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

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

<pre>
<strong>Input:</strong> low = 3, high = 3, zero = 1, one = 1
<strong>Output:</strong> 8
<strong>Explanation:</strong> 
One possible valid good string is &quot;011&quot;. 
It can be constructed as follows: &quot;&quot; -&gt; &quot;0&quot; -&gt; &quot;01&quot; -&gt; &quot;011&quot;. 
All binary strings from &quot;000&quot; to &quot;111&quot; are good strings in this example.
</pre>

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

<pre>
<strong>Input:</strong> low = 2, high = 3, zero = 1, one = 2
<strong>Output:</strong> 5
<strong>Explanation:</strong> The good strings are &quot;00&quot;, &quot;11&quot;, &quot;000&quot;, &quot;110&quot;, and &quot;011&quot;.
</pre>

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

<ul>
	<li><code>1 &lt;= low&nbsp;&lt;= high&nbsp;&lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= zero, one &lt;= low</code></li>
</ul>
Dynamic Programming
class Solution {
    private static final int MOD = (int) 1e9 + 7;
    private int[] f;
    private int lo;
    private int hi;
    private int zero;
    private int one;

    public int countGoodStrings(int low, int high, int zero, int one) {
        f = new int[high + 1];
        Arrays.fill(f, -1);
        lo = low;
        hi = high;
        this.zero = zero;
        this.one = one;
        return dfs(0);
    }

    private int dfs(int i) {
        if (i > hi) {
            return 0;
        }
        if (f[i] != -1) {
            return f[i];
        }
        long ans = 0;
        if (i >= lo && i <= hi) {
            ++ans;
        }
        ans += dfs(i + zero) + dfs(i + one);
        ans %= MOD;
        f[i] = (int) ans;
        return f[i];
    }
}