An array is a data structure that holds a fixed number of objects. Because arrays have fixed sizes, they are highly efficient for quick lookups regardless of how big the array is. However, there is a tradeoff with this fast access time; any insertion or deletion from the middle of the array requires moving the rest of the elements to fill in or close the gap. To optimize time efficiency, try to add and delete mostly from the end of the array.
Arrays commonly come up in interviews, so it's important to review the array library for the language you code in.
Strings are a special kind of array, one that only contains characters. They commonly come up in interview questions, so it's important to go through the string library for the language you're most comfortable with. You should know common operations such as: getting the length, getting a substring, splitting a string based on a delimiter, etc.
It's important to note that whenever you manipulate a string, a new copy of the string is created. There are different ways to reduce the space utilized depending on the language: