Design a data structure that behaves like a stack but also supports retrieving and removing the current maximum value. Implement a class `MaxStack` with the following operations: - `push(x)`: Push integer `x` onto the stack. - `pop()`: Remove and return the top element. - `top()`: Return the top element without removing it. - `peekMax()`: Return the maximum element currently in the stack without removing it. - `popMax()`: Remove and return the maximum element currently in the stack. If there are multiple occurrences of the maximum value, remove the one closest to the top of the stack. All operations should be efficient (better than linear time when possible).