Determine if a given sequence of numbers could have been pushed and popped from a stack, resulting in a second given sequence. Essentially, you need to simulate stack operations and validate the pop order.

Calculate the result of an arithmetic expression given in Reverse Polish Notation (RPN), also known as postfix notation. You'll need to evaluate operations based on the order they appear in the RPN expression.

Determine the total number of continuous subarrays within a given array that sum up to a specific target value, k.