Pfthb

I understand the basics of this problem however I need help on how I can do this the most efficient way possible (taking the least amount of time for the programmer however not substituting stability of the code or efficiency).

Let's say we have a string:

grades=str(input("Enter a string"))

in my code, I would join a space between all characters in the string above and then split the characters into separate items in the same list:

grades=" ".join(grades)
grades.split(" ")

I then want to use loops of some sort to search the list for repeating items. However, I want to learn how I can do this the most efficient way possible:

x=len(grades)
for i in range(0, x):
 if grades[i] == # here is were I'm having trouble

I want to know how I can search whether 1 item in the list is equal to any item in the whole list itself. Kind regards.

I make an example:

from collections import Counter

a =[1,2,3,4,1,2]
c = Counter(a)
for k,v in c.items():
 if v>1:
 print(k,'repeated more than once')

Here the c will be a Counter object like this Counter(1: 2, 2: 2, 3: 1, 4: 1). the keys are the array values and values are the count of them.
So I write the for for your understanding. You can do anything with c, it acts like a dict.

 >> [k for k,v in c.items() if v>1]
 [1, 2]