Python/other/nested_brackets.py

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Python
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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
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.
'''
def is_balanced(S):
stack = []
for i in range(len(S)):
if S[i] == '(' or S[i] == '{' or S[i] == '[':
stack.append(S[i])
else:
if len(stack) > 0:
pair = stack.pop() + S[i]
if pair != '[]' and pair != '()' and pair != '{}':
return False
else:
return False
if len(stack) == 0:
return True
return False
def main():
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S = input("Enter sequence of brackets: ")
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if is_balanced(S):
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print(S, "is balanced")
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else:
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print(S, "is not balanced")
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if __name__ == "__main__":
main()