z.c

size_t l = 0, r = 0;
for (size_t i = 1; i < n; ++i) {
	size_t val = 0;
	if (i < r)
		val = min(r - i, z[i - l]);
	while (i + val < n && s[val] == s[i + val])
		++val;
	z[i] = val;
	if (r < i + val) {
		r = i + val;
		l = i;
	}
}