List Comprehension

List comprehension is a neat was to constuct a list in python within one line of code. The code below shows the general syntax for appending items using a loop.

for item in iterable:
  if condition == True:
    newlist.append(x)

By using list comprehension, this code can be reduced to:

newlist = [expression for item in iterable if condition == True]

A solution to this problem using list comprehension is the funtion firstUniqChar(s) shown below:

def firstUniqChar(s):
        """
        :type s: str
        :rtype: int
        """
        
        if len(s) == 0:
            return -1

        elif len(s) == 1:
            return 0

        else:
            idx = [s.index(x) for x in 'abcdefghijklmnopqrstuvwxyz' if s.count(x) == 1]
            return min(idx)

The funtion firstUniqChar(s) takes a string s as the input. If the length of the string is 0 or 1 then the solution is trivial. In the case when there are multiple letters in the string, we use list comprehension to loop through each letter of the alphabet x in 'abcdefghijklmnopqrstuvwxyz' and keep the indices s.index(x) where the letter only occurs once s.count(x) == 1. However, since we are traversing the letters in alphabetical order, we need to take the minimum index as the solution return min(idx).

firstUniqChar(s = 'aabbdc')

In this example, the indices which are unique are returned as follows idx = [5,4]. By taking the minimum of this list we gauruntee to return the first unique letter d, even though its not the first unique intex returned.