itouhhh… - The Daily WTF
Frequently in programming, we can make a tradeoff: use less (or more) CPU in exchange for using more (or less) memory. Lookup tables are a great example: use a big pile of memory to turn complicated calculations into O(1) operations.
So, for example, implementing itoa, the C library function for turning an integer into a character array (aka, a string), you could maybe make it more efficient using a lookup table.