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Código

#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <queue>
#include <set>
#include <map>
#include <vector>
#include <stack>
#include <algorithm>
#define forn(a, n) for(int a = 0; a<(int) (n); ++a)
#define rforn(a, n) for(int a = (n)-1; a>=0; --a)
using namespace std;
const int N = 6e5+20;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    
    string s;
    cin>>s;
    vector <int> v (s.size(), 0);

    if(s.size() == 0 || s.size() == 1) { cout<<0<<endl; return 0; }
    int borde = 0;
    for (int i = 1; i < s.size(); i++)
    {
        while(s[i] != s[borde] && borde > 0) borde = v[borde-1];
        if (s[i] == s[borde]) borde++;
        v[i] = borde;
    }
    cout<<v[s.size()-1]<<endl;  
    return 0;
}