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Algorithms

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

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

<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>

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

&nbsp;
Example 1:

<pre>
Input
[&quot;MinStack&quot;,&quot;push&quot;,&quot;push&quot;,&quot;push&quot;,&quot;getMin&quot;,&quot;pop&quot;,&quot;top&quot;,&quot;getMin&quot;]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
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>

&nbsp;
Constraints:

<ul>
	<li><code>-231 &lt;= val &lt;= 231 - 1</code></li>
	<li>Methods <code>pop</code>, <code>top</code> and <code>getMin</code> operations will always be called on non-empty stacks.</li>
	<li>At most <code>3 * 104</code> calls will be made to <code>push</code>, <code>pop</code>, <code>top</code>, and <code>getMin</code>.</li>
</ul>