Lowest lexicographical string

Riya wants to learn various new words to improve her vocabulary. Instead of learning all the words one by one, she discovered a new technique to learn. She concatenated all the words together to form a new string such that its lexicographical order is minimum. e.g. Words air, water, bird, land, meerut, meet, lead would become airbirdlandleadmeerutmeetwater Based on her approach, you are handed t