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51 lines
1.3 KiB
Python
51 lines
1.3 KiB
Python
'''
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The nested brackets problem is a problem that determines if a sequence of
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brackets are properly nested. A sequence of brackets s is considered properly nested
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if any of the following conditions are true:
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- s is empty
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- s has the form (U) or [U] or {U} where U is a properly nested string
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- 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.
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The function called is_balanced takes as input a string S which is a sequence of brackets and
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returns true if S is nested and false otherwise.
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'''
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from __future__ import print_function
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def is_balanced(S):
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stack = []
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open_brackets = set({'(', '[', '{'})
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closed_brackets = set({')', ']', '}'})
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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():
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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__":
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main()
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