Latest Time by Replacing Hidden Digits
<p>You are given a string <code>time</code> in the form of <code> hh:mm</code>, where some of the digits in the string are hidden (represented by <code>?</code>).</p>

<p>The valid times are those inclusively between <code>00:00</code> and <code>23:59</code>.</p>

<p>Return <em>the latest valid time you can get from</em> <code>time</code><em> by replacing the hidden</em> <em>digits</em>.</p>

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

<pre>
<strong>Input:</strong> time = &quot;2?:?0&quot;
<strong>Output:</strong> &quot;23:50&quot;
<strong>Explanation:</strong> The latest hour beginning with the digit &#39;2&#39; is 23 and the latest minute ending with the digit &#39;0&#39; is 50.
</pre>

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

<pre>
<strong>Input:</strong> time = &quot;0?:3?&quot;
<strong>Output:</strong> &quot;09:39&quot;
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> time = &quot;1?:22&quot;
<strong>Output:</strong> &quot;19:22&quot;
</pre>

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

<ul>
	<li><code>time</code> is in the format <code>hh:mm</code>.</li>
	<li>It is guaranteed that you can produce a valid time from the given string.</li>
</ul>
Greedy; String
class Solution {
public:
    string maximumTime(string time) {
        if (time[0] == '?') {
            time[0] = (time[1] >= '4' && time[1] <= '9') ? '1' : '2';
        }
        if (time[1] == '?') {
            time[1] = (time[0] == '2') ? '3' : '9';
        }
        if (time[3] == '?') {
            time[3] = '5';
        }
        if (time[4] == '?') {
            time[4] = '9';
        }
        return time;
    }
};