The following code displays all permutations of
the string "eat":
vector<string> v = generate_permutations("eat");
for(int i = 0; i < v.size(); i++)
cout << v[i] << "\n";
To generate the permutations recursively,
generate all permutations that state with the letter 'e', the those
that start with 'a', then those that start with 't'.
Use the recursion to generate all the
permutations of the shorter (two-letter) strings.