【Trivial】Brute force whether "a + b == credit" for a, b in list of items
【Trivial】Simply reverse the list of words
【Easy】Build a map from letters to digits, and use it
【Medium】Find out first Ken's optimal strategy and then Naomi's optimal deceptive strategy
【Trivial】Mark all the occurred digits
【Easy】Recursively flip the pancakes
【Easy】After failures at an is_prime() function, observe the solution pattern and use a magic number as the stop figure.