from collections import defaultdict
def FindElementThatAppearsOnceInArray(lst):
# initialize a dictionary that will assign a
# default value to non-existent keys
dict = defaultdict(int)
for x in lst:
dict[x] += 1
for x, freq in dict.items():
if freq == 1:
return x
return -1
lst = [3, 5, 5, 2, 7, 3, 2, 8, 8, 3, 2, 5, 8]
print(FindElementThatAppearsOnceInArray(lst))
'''
run:
7
'''