Envío 2453

Código

#include <iostream>
#include <bits/stdc++.h> 
#include <vector>
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);
	return 0;
}