1. 직접 구현

import sys
n = int(sys.stdin.readline())
# 이건 느림. n = int(input())
stack = []

for i in range(n):
	command = sys.stdin.readline().split()
  # 이건 느림. command = input().split()

  if command[0] == 'push':
    x = int(command[1])
    stack.append(x)
  
  if command[0] == 'pop':
    if not stack:
        print(-1)
    else:
        print(stack[-1])
        stack.pop()

  if command[0] == 'size':
    print(len(stack))
  
  if command[0] == 'empty':
    if not stack:
        print(1)
    else:
        print(0)
  
  if command[0] == 'top':
    if not stack:
        print(-1)
    else:
        print(stack[-1])

1. 함수 만들어 사용해보기

import sys

def push(x):
  stack.append(x)

def pop():
  if not stack:
    return -1
  return stack.pop()

def size():
  return len(stack)

def empty():
  if not stack:
    return 1
  return 0

def top():
  if not stack:
    return -1
  return stack[-1]

n = int(sys.stdin.readline())
stack = []

for i in range(n):
  command = sys.stdin.readline().split()

  if command[0] == 'push':
    x = int(command[1])
    push(x)
  
  if command[0] == 'pop':
    print(pop())

  if command[0] == 'size':
    print(size())
  
  if command[0] == 'empty':
    print(empty())
  
  if command[0] == 'top':
    print(top())