Micro-Drill #255 — Frequency count (first unique)

Hash Strategies Target: 10s

Two-pass frequency count: first pass counts, second pass finds the target. Works for first unique, most common, etc.

freq = {}
for c in s:
    freq[c] = freq.get(c, 0) + 1
for i, c in enumerate(s):
    if freq[c] == 1:
        return i

Type it from memory. Go.