Max Difference You Can Get From Changing an Integer
<p>You are given an integer <code>num</code>. You will apply the following steps to <code>num</code> <strong>two</strong> separate times:</p>

<ul>
	<li>Pick a digit <code>x (0 &lt;= x &lt;= 9)</code>.</li>
	<li>Pick another digit <code>y (0 &lt;= y &lt;= 9)</code>. Note <code>y</code> can be equal to <code>x</code>.</li>
	<li>Replace all the occurrences of <code>x</code> in the decimal representation of <code>num</code> by <code>y</code>.</li>
</ul>

<p>Let <code>a</code> and <code>b</code> be the two results from applying the operation to <code>num</code> <em>independently</em>.</p>

<p>Return <em>the max difference</em> between <code>a</code> and <code>b</code>.</p>

<p>Note that neither <code>a</code> nor <code>b</code> may have any leading zeros, and <strong>must not</strong> be 0.</p>

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

<pre>
<strong>Input:</strong> num = 555
<strong>Output:</strong> 888
<strong>Explanation:</strong> The first time pick x = 5 and y = 9 and store the new integer in a.
The second time pick x = 5 and y = 1 and store the new integer in b.
We have now a = 999 and b = 111 and max difference = 888
</pre>

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

<pre>
<strong>Input:</strong> num = 9
<strong>Output:</strong> 8
<strong>Explanation:</strong> The first time pick x = 9 and y = 9 and store the new integer in a.
The second time pick x = 9 and y = 1 and store the new integer in b.
We have now a = 9 and b = 1 and max difference = 8
</pre>

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

<ul>
	<li><code>1 &lt;= num &lt;= 10<sup>8</sup></code></li>
</ul>
Greedy; Math
function maxDiff(num: number): number {
    let a = num.toString();
    let b = a;
    for (let i = 0; i < a.length; ++i) {
        if (a[i] !== '9') {
            a = a.split(a[i]).join('9');
            break;
        }
    }
    if (b[0] !== '1') {
        b = b.split(b[0]).join('1');
    } else {
        for (let i = 1; i < b.length; ++i) {
            if (b[i] !== '0' && b[i] !== '1') {
                b = b.split(b[i]).join('0');
                break;
            }
        }
    }
    return +a - +b;
}
