Find the Missing Engineer

Problem:

The SearchEngineerCorporation was throwing a big party for all of it’s engineers for being the most popular search engine used. All engineers had to scan their ID’s on their way in and out of the party, however, all but one engineer scanned out after scanning in, could you try to find out the ID number of the engineer who never made it out of the party?

Given an array of integers, only 1 integer appears once. Find that integer.

Examples:

					
#If given:
[15, 6, 7, 2, 10, 15, 2, 6, 7, 15, 2, 15, 2]

#Engineer 10 didn’t scan out!
					
				

Note:
Engineers are allowed to leave and come back to the party multiple times, so every engineers ID number shows up as a multiple of 2 except for one engineer whose ID number only shows once.

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