<h2><a href="https://leetcode.com/problems/min-stack/">155. Min Stack</a></h2><h3>Medium</h3><hr><div><p>Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.</p>

<p>Implement the <code>MinStack</code> class:</p>

<ul>
	<li><code>MinStack()</code> initializes the stack object.</li>
	<li><code>void push(int val)</code> pushes the element <code>val</code> onto the stack.</li>
	<li><code>void pop()</code> removes the element on the top of the stack.</li>
	<li><code>int top()</code> gets the top element of the stack.</li>
	<li><code>int getMin()</code> retrieves the minimum element in the stack.</li>
</ul>

<p>You must implement a solution with <code>O(1)</code> time complexity for each function.</p>

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

<pre><strong>Input</strong>
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

<strong>Output</strong>
[null,null,null,null,-3,null,0,-2]

<strong>Explanation</strong>
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2
</pre>

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

<ul>
	<li><code>-2<sup>31</sup> &lt;= val &lt;= 2<sup>31</sup> - 1</code></li>
	<li>Methods <code>pop</code>, <code>top</code> and <code>getMin</code> operations will always be called on <strong>non-empty</strong> stacks.</li>
	<li>At most <code>3 * 10<sup>4</sup></code> calls will be made to <code>push</code>, <code>pop</code>, <code>top</code>, and <code>getMin</code>.</li>
</ul>
</div>