Package hw1;

import java. util. TreeMap;

public class HW1 {

/**

* Inverts an array of Strings by returning a TreeMap that maps Strings to the smallest index in the array that contains the String.

*

* @param a an array of Strings

* @return a Map that given a String returns the smallest index of the array that contains the String

* (or null if String is not in the array a).

*/

public static TreeMap invert (String[] a) {

//TO DO

return null;

}

/**

* Computes the total number of occurrences of every String in an array.

*

* @param a an array of Strings

* @return a Map that given a String returns the number of occurrences of that String in the array

* (or null if String is not in the array a).

*/

public static TreeMap count (String[] a) {

/ / TODO

return null;

}

}

The Java code is given below with appropriate comments in key areas of the code

Explanation:

import java. util. TreeMap;

public class HW1 {

/**

* Inverts an array of Strings by returning a TreeMap that maps Strings to the

* smallest index in the array that contains the String.

*

* @param a an array of Strings

* @return a Map that given a String returns the smallest index of the array

* that contains the String (or null if String is not in the array a).

*/

public static TreeMap invert (String[] a) {

TreeMap tmap = new TreeMap ();

for (int i = 0; i < a. length; i++) {

if (! tmap. containsKey (a[i])) {

tmap. put (a[i], i);

}

}

return tmap;

}

/**

* Computes the total number of occurrences of every String in an array.

*

* @param a an array of Strings

* @return a Map that given a String returns the number of occurrences of that

* String in the array (or null if String is not in the array a).

*/

public static TreeMap count (String[] a) {

TreeMap tmap = new TreeMap ();

for (int i = 0; i < a. length; i++) {

tmap. put (a[i], 1 + tmap. getOrDefault (a[i], 0));

}

return tmap;

}

}