Envío 2333

Código

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

using namespace std;

vector<int> ans(string &re){
    vector<int> kmp(re.size(),0);
    int border = 0;
    for(int i = 1; i < re.size();++i){
        while(border && re[i] != re[border])
            border = kmp[border-1];
        if(re[i] == re[border])
            ++border;
        kmp[i] = border;
    }
    return kmp;
}

int main(){
    string re;
    cin >> re;
    vector<int> kmp = ans(re);
    printf("%d\n",kmp.back());
    return 0;
}