Python/other/nested_brackets.py

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"""
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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
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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.
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"""
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def is_balanced(S):
stack = []
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open_brackets = set({"(", "[", "{"})
closed_brackets = set({")", "]", "}"})
open_to_closed = dict({"{": "}", "[": "]", "(": ")"})
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for i in range(len(S)):
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if S[i] in open_brackets:
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stack.append(S[i])
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elif S[i] in closed_brackets:
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if len(stack) == 0 or (
len(stack) > 0 and open_to_closed[stack.pop()] != S[i]
):
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return False
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return len(stack) == 0
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def main():
s = input("Enter sequence of brackets: ")
if is_balanced(s):
print(s, "is balanced")
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else:
print(s, "is not balanced")
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if __name__ == "__main__":
main()