nested_brackets.py

'''
The nested brackets problem is a problem that determines if a sequence of
brackets are properly nested.  A sequence of brackets s is considered properly nested
if any of the following conditions are true:

	- s is empty
	- s has the form (U) or [U] or {U} where U is a properly nested string
	- s has the form VW where V and W are properly nested strings

For example, the string "()()[()]" is properly nested but "[(()]" is not.

The function called is_balanced takes as input a string S which is a sequence of brackets and
returns true if S is nested and false otherwise.

'''
from __future__ import print_function


def is_balanced(S):

    stack = []
    open_brackets = set({'(', '[', '{'})
    closed_brackets = set({')', ']', '}'})
    open_to_closed = dict({'{':'}', '[':']', '(':')'})

    for i in range(len(S)):

        if S[i] in open_brackets:
            stack.append(S[i])

        elif S[i] in closed_brackets:
            if len(stack) == 0 or (len(stack) > 0 and open_to_closed[stack.pop()] != S[i]):
                return False

    return len(stack) == 0


def main():

    S = input("Enter sequence of brackets: ")

    if is_balanced(S):
        print((S, "is balanced"))

    else:
        print((S, "is not balanced"))


if __name__ == "__main__":
    main()