How to find the longest border of a string?

The border of a string is a substring which is both a proper prefix and proper suffix of the string — "proper" meaning that the whole string does not count as a substring. The longest border of x is "ab". The longest border of y is "abab" (the prefix and suffix can overlap).

More details can be found at the link: https://en.wikipedia.org/wiki/Substring#Border

e.g

abcdabc - border string is abc - length is 3

ababab - border string is abab - length is 4

C++ solution/ Algorithm for border string is as below.

#include <string>
#include <vector>
#include <iostream>

using namespace std;

int longestBorder(const string& s) {
	int len = s.length();
	vector<int> prefixFunc(len); 
	prefixFunc[0] = 0;
 	 
	int curBorderLen = 0;	
	for (int i = 1; i < len; ++i) {	
		while (curBorderLen > 0 && s[curBorderLen] != s[i]) 
			curBorderLen = prefixFunc[curBorderLen - 1]; 
		 
		if (s[curBorderLen] == s[i]) 
 ++curBorderLen;
 
 		prefixFunc[i] = curBorderLen;
 	}

 	return prefixFunc[len-1];
}

int main() {
	string s;
	cin >> s;
	cout << longestBorder(s) << endl;
}

Time Complexity: O(n)

Ideone - https://ideone.com/UIjwSG