Python/strings/manacher.py

53 lines
1.8 KiB
Python
Raw Normal View History

2019-05-14 17:15:53 +00:00
# calculate palindromic length from center with incrementing difference
def palindromic_length( center, diff, string):
if center-diff == -1 or center+diff == len(string) or string[center-diff] != string[center+diff] :
return 0
return 1 + palindromic_length(center, diff+1, string)
def palindromic_string( input_string ):
"""
Manachers algorithm which finds Longest Palindromic Substring in linear time.
1. first this conver input_string("xyx") into new_string("x|y|x") where odd positions are actual input
characters.
2. for each character in new_string it find corresponding length and store,
a. max_length
b. max_length's center
3. return output_string from center - max_length to center + max_length and remove all "|"
"""
max_length = 0
# if input_string is "aba" than new_input_string become "a|b|a"
new_input_string = ""
output_string = ""
# append each character + "|" in new_string for range(0, length-1)
for i in input_string[:len(input_string)-1] :
new_input_string += i + "|"
#append last character
new_input_string += input_string[-1]
# for each character in new_string find corresponding palindromic string
for i in range(len(new_input_string)) :
# get palindromic length from ith position
length = palindromic_length(i, 1, new_input_string)
# update max_length and start position
if max_length < length :
max_length = length
start = i
#create that string
for i in new_input_string[start-max_length:start+max_length+1] :
if i != "|":
output_string += i
return output_string
if __name__ == '__main__':
n = input()
print(palindromic_string(n))