How can I find a number which occurs an odd number of times in a SORTED array in O(n) time?

Question

I have a question and I tried to think over it again and again... but got nothing so posting the question here. Maybe I could get some view-point of others, to try and make it w

Answer 1

Use a hash table

For each element E in the input set
    if E is set in the hash table
         increment it's value
    else        
         set E in the hash table and initialize it to 0

For each key K in hash table
    if K % 2 = 1
        return K

As this algorithm is 2n it belongs to O(n)