Know your containers

std::unordered_map is your friend in this problem. Whenever you've never previously seen a number, simply use the operator[] or insert function to add the number and its index. When using find, it will return an iterator, which is a key-value pair.

eg: auto location = m.find(numToFind);

location->first is your key, and location->second is your value

When you return, don't use push_back

You can simply return an initializer list like: {i,j}.