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53 lines
1.8 KiB
Python
53 lines
1.8 KiB
Python
# calculate palindromic length from center with incrementing difference
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def palindromic_length( center, diff, string):
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if center-diff == -1 or center+diff == len(string) or string[center-diff] != string[center+diff] :
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return 0
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return 1 + palindromic_length(center, diff+1, string)
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def palindromic_string( input_string ):
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"""
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Manacher’s algorithm which finds Longest Palindromic Substring in linear time.
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1. first this conver input_string("xyx") into new_string("x|y|x") where odd positions are actual input
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characters.
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2. for each character in new_string it find corresponding length and store,
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a. max_length
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b. max_length's center
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3. return output_string from center - max_length to center + max_length and remove all "|"
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"""
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max_length = 0
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# if input_string is "aba" than new_input_string become "a|b|a"
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new_input_string = ""
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output_string = ""
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# append each character + "|" in new_string for range(0, length-1)
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for i in input_string[:len(input_string)-1] :
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new_input_string += i + "|"
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#append last character
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new_input_string += input_string[-1]
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# for each character in new_string find corresponding palindromic string
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for i in range(len(new_input_string)) :
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# get palindromic length from ith position
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length = palindromic_length(i, 1, new_input_string)
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# update max_length and start position
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if max_length < length :
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max_length = length
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start = i
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#create that string
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for i in new_input_string[start-max_length:start+max_length+1] :
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if i != "|":
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output_string += i
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return output_string
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if __name__ == '__main__':
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n = input()
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print(palindromic_string(n))
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