Micro-Drill #254 — Complement map (two sum)

Hash Strategies Target: 10s

Store what you've seen, check for the complement. Turns O(n^2) brute force into O(n). The fundamental hash map pattern.

seen = {}
for i, x in enumerate(a):
    if target - x in seen:
        return [seen[target - x], i]
    seen[x] = i

Type it from memory. Go.