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
class Solution {
    public int maxDiff(int num) {
        String a = String.valueOf(num);
        String b = a;
        for (int i = 0; i < a.length(); ++i) {
            if (a.charAt(i) != '9') {
                a = a.replace(a.charAt(i), '9');
                break;
            }
        }
        if (b.charAt(0) != '1') {
            b = b.replace(b.charAt(0), '1');
        } else {
            for (int i = 1; i < b.length(); ++i) {
                if (b.charAt(i) != '0' && b.charAt(i) != '1') {
                    b = b.replace(b.charAt(i), '0');
                    break;
                }
            }
        }
        return Integer.parseInt(a) - Integer.parseInt(b);
    }
}