Your task is to produce a map of the frequencies of all letters in a string. Letters are represented as strings of length 1. For example, if you are given a string

Mary had a little lamb

you should produce the map

" " -> 4
"M" -> 1
"a" -> 4
"b" -> 1
"d" -> 1
"e" -> 1
"h" -> 1 
"i" -> 1
"l" -> 3
"m" -> 1
"r" -> 1
"t" -> 2
"y" -> 1

Construct a map that contains the keys in sorted order.

Hint: Use autoboxing:

Integer f = freqs.get(letter); // Caution: may be null
. . .
freqs.put(letter, f + 1); // Can use arithmetic with Integer

Complete the following file:

LetterMap.java

import java.util.Map; import java.util.TreeMap; public class LetterMap { public Map<String, Integer> makeLetterMap(String str) { // TODO: Complete method } }