【模板】扩展kmp
vector<int> z_function(string s) {
int n = (int)s.length();
vector<int> z(n);
for (int i = 1, l = 0, r = 0; i < n; ++i) {
if (i <= r) z[i] = min(r - i + 1, z[i - l]);
while (i + z[i] < n && s[z[i]] == s[i + z[i]]) ++z[i];
if (i + z[i] - 1 > r) l = i, r = i + z[i] - 1;
}
return z;
}