<p>You are keeping score for a baseball game with strange rules. The game consists of several rounds, where the scores of past rounds may affect future rounds' scores.</p>
<p>At the beginning of the game, you start with an empty record. You are given a list of strings <code>ops</code>, where <code>ops[i]</code> is the <code>i<sup>th</sup></code> operation you must apply to the record and is one of the following:</p>
<ol>
<li>An integer <code>x</code> - Record a new score of <code>x</code>.</li>
<li><code>"+"</code> - Record a new score that is the sum of the previous two scores. It is guaranteed there will always be two previous scores.</li>
<li><code>"D"</code> - Record a new score that is double the previous score. It is guaranteed there will always be a previous score.</li>
<li><code>"C"</code> - Invalidate the previous score, removing it from the record. It is guaranteed there will always be a previous score.</li>
<li><code>ops[i]</code> is <code>"C"</code>, <code>"D"</code>, <code>"+"</code>, or a string representing an integer in the range <code>[-3 * 10<sup>4</sup>, 3 * 10<sup>4</sup>]</code>.</li>
<li>For operation <code>"+"</code>, there will always be at least two previous scores on the record.</li>
<li>For operations <code>"C"</code> and <code>"D"</code>, there will always be at least one previous score on the record.</li>
