leetcode-problemset/leetcode-cn/problem (English)/用栈操作构建数组(English) [build-an-array-with-stack-operations].html

<p>You are given an integer array <code>target</code> and an integer <code>n</code>.</p>

<p>You have an empty stack with the two following operations:</p>

<ul>
	<li><strong><code>&quot;Push&quot;</code></strong>: pushes an integer to the top of the stack.</li>
	<li><strong><code>&quot;Pop&quot;</code></strong>: removes the integer on the top of the stack.</li>
</ul>

<p>You also have a stream of the integers in the range <code>[1, n]</code>.</p>

<p>Use the two stack operations to make the numbers in the stack (from the bottom to the top) equal to <code>target</code>. You should follow the following rules:</p>

<ul>
	<li>If the stream of the integers is not empty, pick the next integer from the stream and push it to the top of the stack.</li>
	<li>If the stack is not empty, pop the integer at the top of the stack.</li>
	<li>If, at any moment, the elements in the stack (from the bottom to the top) are equal to <code>target</code>, do not read new integers from the stream and do not do more operations on the stack.</li>
</ul>

<p>Return <em>the stack operations needed to build </em><code>target</code> following the mentioned rules. If there are multiple valid answers, return <strong>any of them</strong>.</p>

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

<pre>
<strong>Input:</strong> target = [1,3], n = 3
<strong>Output:</strong> [&quot;Push&quot;,&quot;Push&quot;,&quot;Pop&quot;,&quot;Push&quot;]
<strong>Explanation:</strong> Initially the stack s is empty. The last element is the top of the stack.
Read 1 from the stream and push it to the stack. s = [1].
Read 2 from the stream and push it to the stack. s = [1,2].
Pop the integer on the top of the stack. s = [1].
Read 3 from the stream and push it to the stack. s = [1,3].
</pre>

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

<pre>
<strong>Input:</strong> target = [1,2,3], n = 3
<strong>Output:</strong> [&quot;Push&quot;,&quot;Push&quot;,&quot;Push&quot;]
<strong>Explanation:</strong> Initially the stack s is empty. The last element is the top of the stack.
Read 1 from the stream and push it to the stack. s = [1].
Read 2 from the stream and push it to the stack. s = [1,2].
Read 3 from the stream and push it to the stack. s = [1,2,3].
</pre>

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

<pre>
<strong>Input:</strong> target = [1,2], n = 4
<strong>Output:</strong> [&quot;Push&quot;,&quot;Push&quot;]
<strong>Explanation:</strong> Initially the stack s is empty. The last element is the top of the stack.
Read 1 from the stream and push it to the stack. s = [1].
Read 2 from the stream and push it to the stack. s = [1,2].
Since the stack (from the bottom to the top) is equal to target, we stop the stack operations.
The answers that read integer 3 from the stream are not accepted.
</pre>

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

<ul>
	<li><code>1 &lt;= target.length &lt;= 100</code></li>
	<li><code>1 &lt;= n &lt;= 100</code></li>
	<li><code>1 &lt;= target[i] &lt;= n</code></li>
	<li><code>target</code> is strictly increasing.</li>
</ul>