LeetCode-Swift | Solutions to LeetCode by Swift | Learning library

The dict initialised in the method stores the numbers in the input as keys, and their indices as values. The program uses this to remember which number is where. The dict can tell you things like "the number 2 is at index 0".

For each number num at index i in the input array, we subtract num from the target to find the other number that we need, in order for them to add up to target.

Now we have the other number we need, we check to see if we have seen such a number before, by searching dict. This is what the if let lastIndex = dict[target - num] part is doing. If the dict knows what index the other number is at, we return that index, and i.

If we haven't seen that number before, we record i into the dictionary under the key num, hoping that in later iterations, we can find a number that when added to num, makes 9.