Map of Slices

A map value can be anything, and map[K][]V - a key with a list under it - is the shape you reach for whenever you are grouping. Go makes the common case short:

groups[key] = append(groups[key], value)

That works even when key is not in the map yet. The missing value reads as a nil slice, append on a nil slice allocates a new one, and the assignment puts the result back. Three separate rules line up so that grouping needs no "does this key exist" branch.

The assignment is not optional, though. append returns a header that may point at a different array, and the map holds a copy of the header, so append(groups[key], value) on its own throws the result away.

The other half of this challenge is the flip side of the same fact: copying a map copies slice headers, not arrays. Two maps built that way share their elements, and a write through one is visible through the other. A copy is only deep if each slice is copied too.

for k, v := range src {
	dst[k] = append([]string(nil), v...)   // its own array
}

Task

Fill in the seven functions in main.go.

1. Append adds one member to a group, creating the group when it is new and doing nothing at all to a nil map.
2. GroupByFirstLetter buckets words by their lowercased first letter, in input order.
3. Keys returns the group names sorted, and Count totals the members.
4. Clone is a deep copy, and Merge combines two maps into a third that borrows nothing from either.
5. Invert turns groups-of-members into members-of-groups, each list sorted.

Hints

• Writing to a nil map panics, so Append needs an explicit if groups == nil guard before it touches anything.
• word[:1] is the first byte of a string, which is enough here because every test word starts with an ASCII letter.
• append([]string(nil), members...) is the shortest deep copy of one group. slices.Clone does the same thing if you would rather import it.
• Merge is Clone plus a loop. Cloning first is what keeps it from appending into a caller's spare capacity - and there is a test for exactly that.
• Invert gets sorted lists for free if you range over Keys(groups) rather than over the map.