Implement FreqStack, a class which simulates the operation of a stack-like data structure.

FreqStack has two functions:

push(int x), which pushes an integer x onto the stack. pop(), which removes and returns the most frequent element in the stack.
If there is a tie for most frequent element, the element closest to the top of the stack is removed and returned.

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: Aftermakingsix .pushoperations, thestackis [5,7,5,7,4,5] frombottomtotop. Then: pop() ->returns5, as5isthemostfrequent. Thestackbecomes [5,7,5,7,4]. pop() ->returns7, as5and7isthemostfrequent, but7isclosesttothetop. Thestackbecomes [5,7,5,4]. pop() ->returns5.Thestackbecomes [5,7,4]. pop() ->returns4.Thestackbecomes [5,7].

Note:

• Calls to FreqStack.push(int x) will be such that 0 <= x <= 10^9.
• It is guaranteed that FreqStack.pop() won't be called if the stack has zero elements.
• The total number of FreqStack.push calls will not exceed 10000 in a single test case.
• The total number of FreqStack.pop calls will not exceed 10000 in a single test case.
• The total number of FreqStack.push and FreqStack.pop calls will not exceed 150000 across all test cases.

实现 FreqStack，模拟类似栈的数据结构的操作的一个类。

FreqStack 有两个函数：

• push(int x)，将整数 x 推入栈中。
• pop()，它移除并返回栈中出现最频繁的元素。如果最频繁的元素不只一个，则移除并返回最接近栈顶的元素。

FreqStack 里面保存频次的 map 和相同频次 group 的 map。push 的时候动态的维护 x 的频次，并更新到对应频次的 group 中。pop 的时候对应减少频次字典里面的频次，并更新到对应频次的 group 中。