Two Sum — Interview Question Guide

• Complement lookup: for each x, you need target - x; store seen values in a hash map.
• Single-pass vs two-pass: a single pass that checks the map before inserting avoids double-counting an element.
• Hash maps give O(1) average lookup, which is what makes the optimal solution O(n).

1. Create an empty hash map from value to index.
2. Iterate the array; for each element compute its complement.
3. If the complement is already in the map, return both indices.
4. Otherwise store the current value and its index and continue.

def two_sum(nums, target):
    seen = {}
    for i, x in enumerate(nums):
        need = target - x
        if need in seen:
            return [seen[need], i]
        seen[x] = i