Best way to find first non repeating character in a string

Question

What might be the best space and time-efficient productive answer for finding the primary non-repeating character for a string like aabccbdcbe?

Answer 1

The solution is, O(n):

def fn(s):

  order = []

  counts = {}

  for x in s:

    if x in counts:

      counts[x] += 1

    else:

      counts[x] = 1 

      order.append(x)

  for x in order:

    if counts[x] == 1:

      return x

  return None

We actually loop through the string once. At the point when we go over another character, we store it in checks with an estimation of 1, and add it to order. At the point when we run over a character we've seen previously, we increase its value in counts. At last, we loop through order until we discover a character with an estimation of 1 in checks and returns it back.

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