Baseball Game
<p>You are keeping the scores for a baseball game with strange rules. At the beginning of the game, you start with an empty record.</p>

<p>You are given a list of strings <code>operations</code>, where <code>operations[i]</code> is the <code>i<sup>th</sup></code> operation you must apply to the record and is one of the following:</p>

<ul>
	<li>An integer <code>x</code>.

    <ul>
    	<li>Record a new score of <code>x</code>.</li>
    </ul>
    </li>
    <li><code>&#39;+&#39;</code>.
    <ul>
    	<li>Record a new score that is the sum of the previous two scores.</li>
    </ul>
    </li>
    <li><code>&#39;D&#39;</code>.
    <ul>
    	<li>Record a new score that is the double of the previous score.</li>
    </ul>
    </li>
    <li><code>&#39;C&#39;</code>.
    <ul>
    	<li>Invalidate the previous score, removing it from the record.</li>
    </ul>
    </li>

</ul>

<p>Return <em>the sum of all the scores on the record after applying all the operations</em>.</p>

<p>The test cases are generated such that the answer and all intermediate calculations fit in a <strong>32-bit</strong> integer and that all operations are valid.</p>

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

<pre>
<strong>Input:</strong> ops = [&quot;5&quot;,&quot;2&quot;,&quot;C&quot;,&quot;D&quot;,&quot;+&quot;]
<strong>Output:</strong> 30
<strong>Explanation:</strong>
&quot;5&quot; - Add 5 to the record, record is now [5].
&quot;2&quot; - Add 2 to the record, record is now [5, 2].
&quot;C&quot; - Invalidate and remove the previous score, record is now [5].
&quot;D&quot; - Add 2 * 5 = 10 to the record, record is now [5, 10].
&quot;+&quot; - Add 5 + 10 = 15 to the record, record is now [5, 10, 15].
The total sum is 5 + 10 + 15 = 30.
</pre>

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

<pre>
<strong>Input:</strong> ops = [&quot;5&quot;,&quot;-2&quot;,&quot;4&quot;,&quot;C&quot;,&quot;D&quot;,&quot;9&quot;,&quot;+&quot;,&quot;+&quot;]
<strong>Output:</strong> 27
<strong>Explanation:</strong>
&quot;5&quot; - Add 5 to the record, record is now [5].
&quot;-2&quot; - Add -2 to the record, record is now [5, -2].
&quot;4&quot; - Add 4 to the record, record is now [5, -2, 4].
&quot;C&quot; - Invalidate and remove the previous score, record is now [5, -2].
&quot;D&quot; - Add 2 * -2 = -4 to the record, record is now [5, -2, -4].
&quot;9&quot; - Add 9 to the record, record is now [5, -2, -4, 9].
&quot;+&quot; - Add -4 + 9 = 5 to the record, record is now [5, -2, -4, 9, 5].
&quot;+&quot; - Add 9 + 5 = 14 to the record, record is now [5, -2, -4, 9, 5, 14].
The total sum is 5 + -2 + -4 + 9 + 5 + 14 = 27.
</pre>

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

<pre>
<strong>Input:</strong> ops = [&quot;1&quot;,&quot;C&quot;]
<strong>Output:</strong> 0
<strong>Explanation:</strong>
&quot;1&quot; - Add 1 to the record, record is now [1].
&quot;C&quot; - Invalidate and remove the previous score, record is now [].
Since the record is empty, the total sum is 0.
</pre>

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

<ul>
	<li><code>1 &lt;= operations.length &lt;= 1000</code></li>
	<li><code>operations[i]</code> is <code>&quot;C&quot;</code>, <code>&quot;D&quot;</code>, <code>&quot;+&quot;</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>&quot;+&quot;</code>, there will always be at least two previous scores on the record.</li>
	<li>For operations <code>&quot;C&quot;</code> and <code>&quot;D&quot;</code>, there will always be at least one previous score on the record.</li>
</ul>
Stack; Array; Simulation
func calPoints(operations []string) (ans int) {
	var stk []int
	for _, op := range operations {
		n := len(stk)
		switch op {
		case "+":
			stk = append(stk, stk[n-1]+stk[n-2])
		case "D":
			stk = append(stk, stk[n-1]*2)
		case "C":
			stk = stk[:n-1]
		default:
			num, _ := strconv.Atoi(op)
			stk = append(stk, num)
		}
	}
	for _, x := range stk {
		ans += x
	}
	return
}