Finding duplicate balls in a basket? A basket is full with \$n\$ colored balls. For each color except one, there are at least 2 balls with that color. Find the color which only has one ball. in \$O(n)\$ space complexity.
What if each ball is repeated exactly  twice?
You can consider the input to be an array or a list or any container of colors as strings with a maximum length of 100
This is code-golf so the shortest code in bytes wins.
Python - 17 chars

Solely as an expression:

min(B,key=B.count)


Full, standalone program with input and output, 34 chars:

B=input()
print min(B,key=B.count)


edit: What gnibbler said.