Find the first letter in a string that does not have a pair.

first_positions = zeroes (26)
nb_occs = zeroes (26)
for in range (0,len(s)):
    index = s[i] - '0'
    if nb_occs[index] == 0
       first_positions[index] = i
       nb_occs[index] += 1

then simply iterate on nb_occs and return minimum corresponding value in first_positions where nb_occs == 1