EXPLAIN IN WORDS

Suppose I have a std::vector A. This vector has only values in the range of 0 to A.size(), inclusive. For example, if there are 5 values in A, then the only values it can have are {0,1,2,3,4,5}, although not necessarily in that order. Duplicate values might be present too.

Obviously, at least one value is missing. Using O(n) time and O(n) space, where n is A.size(), determine which value(s) are missing from the vector.