Solve all below question

Question $1$

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left$-$out nodes, in the end, should remain as it is$.$

You may not alter the values in the list's nodes, only nodes themselves may be changed.

Question $2$

What is the Chomsky hierarchy?

Question $3$

Given a string s$,$ return whether s is a valid number.

An integer number followed by an optional exponent.

A decimal number followed by an optional exponent.

The digits are defined as one or more digits.

Question $4$

What is the class EXPTIME?