How could find Number that Appears Only once in array of Duplicates?

Question

For example: It returns ‘b’ when the input is “abaccdeff”.

Answer 1

There are 3-4 ways of solving this question.
1) Solve it in O(n^2). --- :).
2) Use hash table.
3) Do XOR of all the numbers. The final result is the the number that appears once.