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Find the longest sub-string which is prefix, suffix and also present inside the string in Python
Suppose we have a given string, we have to find the largest sub-string which is a prefix, a suffix and a sub-string of that given string. If there is no such substring, then return -1.
So, if the input is like "languagepythonlanguageinterestinglanguage", then the output will be "language"
To solve this, we will follow these steps −
Define a function get_lps() . This will take string
n := size of string
long_pref_suff := an array of size n, and fill with 0
size := 0, long_pref_suff[0] := 0, i := 1
while i < n is non-zero, do
if string[i] is same as string[size], then
size := size + 1
long_pref_suff[i] := size
i := i + 1
otherwise,
if size is not same as 0, then
size := long_pref_suff[size - 1]
otherwise,
long_pref_suff[i] := 0
i := i + 1
return long_pref_suff
From the main method, do the following −
long_pref_suff := get_lps(string)
n := size of string
if long_pref_suff[n - 1] is same as 0, then
return -1
for i in range 0 to n - 1, do
if long_pref_suff[i] is same as long_pref_suff[n - 1], then
return substring of string[from index 0 to long_pref_suff[i]]
if long_pref_suff[long_pref_suff[n - 1] - 1] is same as 0, then
return -1
otherwise,
return substring of string[from index 0 to long_pref_suff[long_pref_suff[n - 1] - 1]]
Example
Let us see the following implementation to get better understanding −
def get_lps(string): n = len(string) long_pref_suff = [0 for i in range(n)] size = 0 long_pref_suff[0] = 0 i = 1 while (i < n): if (string[i] == string[size]): size += 1 long_pref_suff[i] = size i += 1 else: if (size != 0): size = long_pref_suff[size - 1] else: long_pref_suff[i] = 0 i += 1 return long_pref_suff def get_longest_substr(string): long_pref_suff = get_lps(string) n = len(string) if (long_pref_suff[n - 1] == 0): return -1 for i in range(0,n - 1): if (long_pref_suff[i] == long_pref_suff[n - 1]): return string[0:long_pref_suff[i]] if (long_pref_suff[long_pref_suff[n - 1] - 1] == 0): return -1 else: return string[0:long_pref_suff[long_pref_suff[n - 1] - 1]] string = "languagepythonlanguageinterestinglanguage" print(get_longest_substr(string))
Input
"languagepythonlanguageinterestinglanguage"
Output
language
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