You're given a string containing braced groups of comma-separated options. Expand the string to generate all possible combinations by selecting one option from each group.

Convert a string into a 32-bit signed integer. Handle leading whitespace, signs, invalid input, and integer overflow according to the rules of the atoi function.

Given an array of numbers where one number appears more than once, find the duplicate number. You should do this without modifying the original array and using only constant extra space.