Backtracking

We can use backtracking to find all the purchase combinations, and then take the lowest price.

This problem asks us to list all possible combinations, so we can use backtracking to find them. However, we must avoid finding duplicated combinations.

This problem asks us to list all possible combinations of parentheses, so we can use backtracking to solve it.

This problem asks us to list all possible strings, so we can think of using backtracking to solve it.