Implement FreqStack, a class which simulates the operation of a stack-like data structure.
FreqStack has two functions:
<li><code>push(int x)</code>, which pushes an integer <code>x</code> onto the stack.</li>
<li><code>pop()</code>, which <strong>removes</strong> and returns the most frequent element in the stack.
<ul>
<li>If there is a tie for most frequent element, the element closest to the top of the stack is removed and returned.</li>
</ul>
</li>
Example 1:
Input: ["FreqStack","push","push","push","push","push","push","pop","pop","pop","pop"], [[],[5],[7],[5],[7],[4],[5],[],[],[],[]] Output: [null,null,null,null,null,null,null,5,7,5,4] Explanation: After making six .push operations, the stack is [5,7,5,7,4,5] from bottom to top. Then: pop() -> returns 5, as 5 is the most frequent. The stack becomes [5,7,5,7,4]. pop() -> returns 7, as 5 and 7 is the most frequent, but 7 is closest to the top. The stack becomes [5,7,5,4]. pop() -> returns 5. The stack becomes [5,7,4]. pop() -> returns 4. The stack becomes [5,7].
Note:
<li>Calls to <code>FreqStack.push(int x)</code> will be such that <code>0 <= x <= 10^9</code>.</li>
<li>It is guaranteed that <code>FreqStack.pop()</code> won't be called if the stack has zero elements.</li>
<li>The total number of <code>FreqStack.push</code> calls will not exceed <code>10000</code> in a single test case.</li>
<li>The total number of <code>FreqStack.pop</code> calls will not exceed <code>10000</code> in a single test case.</li>
<li>The total number of <code>FreqStack.push</code> and <code>FreqStack.pop</code> calls will not exceed <code>150000</code> across all test cases.</li>