Given a string str. The task is to find the longest sub-string which is a prefix, a suffix and a sub-string of the given string, str. If no such string exists then print -1.
Input: str = “fixprefixsuffix”
“fix” is a prefix, suffix and present inside in the string too.
Input: str = “aaaa”
“aa” is a prefix, suffix and present inside the string.
Approach: Let us calculate the longest prefix suffix for all prefixes of string. longest prefix suffix lps[i] is maximal length of prefix that also is suffix of substring [0…i]. More about longest prefix suffix you can see in a description of kmp algorithm.
The first of possible answers is a prefix of length lps[n-1]. If lps[n-1] = 0, there is no solution. For checking the first possible answer you should iterate over lps[i]. If at least one of them equal to lps[n-1] (but not n-1th, of course) – you found the answer. The second possible answer is a prefix of length lps[lps[n-1]-1]. If lps[lps[n-1]-1] = 0, you also have no solution. Otherwise, you can be sure that the answer already found. This substring is a prefix and a suffix of our string. Also, it is a suffix of a prefix with length lps[n-1] that places inside of all string. This solution works in O(n).
Below is the implementation of the above approach:
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