Stack Calculator

A stack machine processes instructions by pushing and popping values to an internal stack. A simple example of this is a calculator.

The argument passed to run(instructions) will always be a string containing a series of instructions. The instruction set of the calculator will be this:

• +: Pop the last 2 values from the stack, add them, and push the result onto the stack.
• -: Pop the last 2 values from the stack, subtract the lower one from the topmost one, and push the result.
• *: Pop the last 2 values, multiply, and push the result.
• /: Pop the last 2 values, divide the topmost one by the lower one, and push the result.
• DUP: Duplicate (not double) the top value on the stack.
• POP: Pop the last value from the stack and discard it.
• PSH: Performed whenever a number appears as an instruction. Push the number to the stack.
• Any other instruction (for example, a letter) should result in the value "Invalid instruction: [instruction]"

Examples

StackCalc("") ➞ 0

StackCalc("5 6 +") ➞ 11

StackCalc("3 DUP +") ➞ 6

StackCalc("6 5 5 7 * - /") ➞ 5

StackCalc("x y +") ➞ Invalid instruction: x

Notes

• If there are no instructions, the value should remain 0.
• The return value of get value() should be the topmost value on the stack.