I'm making algorithm like crossword, but I dont know how to design the algorithm.

For example:

• there are words like 'car', 'apple' in the dictionary.
• the word 'app' is given on the board.
• there are letters like 'l' 'e' 'c' 'r'....for making words.

So the algorithm's task is to make correct words which are stored in dictionary.

app -> lapp -> leapp -> lecapp -> .... -> lappe -> eappc -> ... -> appl -> apple (correct answer)

What is the best solution for this algorithm?