Finding all integers in an array with odd occurrence
You don't really need to keep track of how many times you've seen each integer, only whether you've seen it an odd number of times. So start with your data structure (map, hash table, dictionary, whatever) being empty, which correctly reflects the state that you have seen no numbers an odd number of times. Now for each number x in the array, if x is in the map then remove it (now you've seen it an even number of times) otherwise add it.