Special Stack with O(1) Minimum Operation

This problem requires you to implement a special stack that supports the standard operations push, pop, top and an additional operation min which retrieves the minimum element in the stack. All operations must run in O(1) time complexity.

The stack should behave as follows:

• push x: Push the integer x onto the stack.
• pop: Remove the top element from the stack and print its value. If the stack is empty, print None.
• top: Print the top element of the stack without removing it. If the stack is empty, print None.
• min: Print the minimum element currently in the stack. If the stack is empty, print None.

For example, consider the following sequence of operations:

push 3
push 5
min
push 2
push 1
min
pop
min
pop
top

The expected output is:

3
1
2
5

Implement the data structure and simulate the operations as described. The input will be read from standard input and output should be printed to standard output.

inputFormat

The first line contains an integer n representing the number of commands. The next n lines each contain a command. A command is one of the following:

• push x: where x is an integer
• pop
• top
• min

There is no output for the push command.

outputFormat

For each pop, top, or min command, output the result on a separate line. If the stack is empty when a pop, top, or min command is executed, output None.

## sample

10
push 3
push 5
min
push 2
push 1
min
pop
min
pop
top

3
1
2
5

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