Complement lookup (Arrays & Hashing Sub-pattern)

// For each element, ask: "is the partner I need already seen?"
vector<int> findPair(vector<int>& nums, int target) {
    unordered_map<int, int> seen;          // value -> index where we saw it
    for (int i = 0; i < (int)nums.size(); i++) {
        int need = target - nums[i];
        auto it = seen.find(need);         // look up FIRST...
        if (it != seen.end()) return {it->second, i};
        seen[nums[i]] = i;                 // ...then remember the current element
    }
    return {};
}