A Stack to use for HackerRank problems.

Stack.py

###
# Python 3
# S = []
###
def size(S):
    return len(S)
def is_empty(S):
    return len(S) == 0
def peek(S):
    return S[-1] if len(S) > 0 else None
def push(S, x):
    S.append(x)
def pop(S):
    return S.pop() if len(S) > 0 else None
# The End