STACK_SIM.py

"""
Perform a sequence of operations over a stack, each element is an integer:
PUSH v: push a value v into the stack 
POP: remove an element out of the stack and print this element to stdout (print NULL if the stack is empty) 
Input:
      Each line contains a command (operration) of type:
        + PUSH v
        + POP 
Output:
      Write the results of POP operations (each result is written in a line)

Example 
Input:
      PUSH 1 
      PUSH 2 
      PUSH 3 
      POP 
      POP 
      PUSH 4 
      PUSH 5 
      POP 
      # 
Output:
      3 
      2 
      5
"""

inp = "."
p = []
while(inp != "#"):
    inp = input()
    if inp=="#":
        break
    if "PUSH" in inp:
        p.append(inp[5:])
    else:
        if len(p) > 0:
            print(p[len(p)-1])
            p.remove(p[len(p)-1])
        else:
            print("NULL")