143 CSE Section Handout #12
Practice Midterm #5
1. ArrayList Mystery. Consider the following method:
public static void mystery5(ArrayList<Integer> list) {
for (int i = 0; i < list.size(); i++) {
int element = list.get(i);
list.remove(i);
list.add(0, element + 1);
}
System.out.println(list);
}
Write the output produced by the method when passed each of the following ArrayLists:
List
Output
(a) [10, 20, 30]
____________________________________
(b) [8, 2, 9, 7, 4]
____________________________________
(c) [-1, 3, 28, 17, 9, 33]
____________________________________
CSE 143 Section Handout #12
2. ArrayList Programming. Write a method filterRange that accepts an ArrayList of integers and two
integer values min and max as parameters and removes all elements whose values are in the range min
through max (inclusive) from the list. For example, if a variable called list stores the values:
[4, 7, 9, 2, 7, 7, 5, 3, 5, 1, 7, 8, 6, 7]
The call of filterRange(list, 5, 7); should remove all values between 5 and 7, therefore it should
change the list to store [4, 9, 2, 3, 1, 8]. If no elements in range min-max are found in the list, the
list's contents are unchanged. If an empty list is passed, the list remains empty. You may assume that the
list is not null.
CSE 143 Section Handout #12
3. Stack and Queue Programming. Write a method removeMin that accepts a stack of integers as a
parameter and removes and returns the smallest value from the stack. For example, if a variable s stores:
bottom [2, 8, 3, 19, 7, 3, 2, 42, 9, 3, 2, 7, 12, -8, 4] top
And we make the following call:
int n = removeMin(s);
The method removes and returns -8, so n will store -8 after the call and s will store the following values:
bottom [2, 8, 3, 19, 7, 3, 2, 42, 9, 3, 2, 7, 12, 4] top
If the minimum value appears more than once, all occurrences of it should be removed. For example, given
the stack above, if we again call removeMin(s);, it would return 2 and would leave the stack as follows:
bottom [8, 3, 19, 7, 3, 42, 9, 3, 7, 12, 4] top
You may use one queue as auxiliary storage. You may not use any other structures to solve this problem,
although you can have as many primitive variables as you like. You may not solve the problem recursively.
You may assume that the stack is not empty. For full credit, your solution must run in O(n) time.
You have access to the following two methods and may call them as needed to help you solve the problem:
public static void s2q(Stack<Integer> s, Queue<Integer> q) {
while (!s.isEmpty()) {
q.add(s.pop());
// Transfers the entire contents
}
// of stack s to queue q
}
public static void q2s(Queue<Integer> q, Stack<Integer> s) {
while (!q.isEmpty()) {
s.push(q.remove());
// Transfers the entire contents
}
// of queue q to stack s
}
CSE 143 Section Handout #12
4. Collections Programming. Write a method whoPassed that determines which students "passed" a course.
The method accepts three parameters: A students map from students' names (strings) to their overall
percentages (integers) in the course; a grades map from percentages (integers) to course grades out of 4.0
(real numbers); and a minGrade real number representing the minimum grade out of 4.0 required to pass.
Your method should return a set containing the names of all students who earned at least the given
minimum grade out of 4.0 (inclusive). For example, if your method is passed the following parameters:
students = {Marty=76, Dan=81, Alyssa=98, Kim=52, Lisa=87, Whit=43, Jeff=70, Sylvia=92}
grades
= {76=2.1, 81=2.6, 98=4.0, 52=0.0, 87=3.3, 43=0.0, 70=1.5, 92=3.7}
minGrade = 2.6
Then your method should return the set [Alyssa, Dan, Lisa, Sylvia], because Alyssa's percentage
of 98 earns her a grade of 4.0 in the course, Dan's 81 earns him a 2.6, Lisa's 87 earns her a 3.3, and Sylvia's
92 earns her a 3.7.
The names can appear in any order in the set. If no students meet the desired minimum grade, return an
empty set. You may assume that no parameter is null, that every student's percentage is between 0 and
100 inclusive, that the grades map contains a grade entry between 0.0 and 4.0 inclusive for every
percentage earned by a student, and that the minGrade parameter is between 0.0 and 4.0 inclusive. For full
credit your code must run in less than O(n2) time where n is the number of students and/or percentages.
CSE 143 Section Handout #12
5. Linked Nodes. Write the code that will turn the "before" picture into the "after" picture by modifying links
between the nodes shown and/or creating new nodes as needed. There may be more than one way to write
the code, but you are NOT allowed to change any existing node's data field value. You also should not
create new ListNode objects unless necessary to add new values to the chain, but you may create a single
ListNode variable to refer to any existing node if you like. If a variable does not appear in the "after"
picture, it doesn't matter what value it has after the changes are made.
To help maximize partial credit in case you make mistakes, we suggest that you include optional comments
with your code that describe the links you are trying to change, as shown in Section 7's solution code.
Before
list
1
2
After
3
4
list
3
2
list2
4
1
Assume that you are using the ListNode class as defined in lecture and section:
public class ListNode {
public int data;
public ListNode next;
}
// data stored in this node
// link a to the next node in the list
public ListNode() { ... }
public ListNode(int data) { ... }
public ListNode(int data, ListNode next) { ... }
CSE 143 Section Handout #12
6. Linked List Programming. Write a method hasAlternatingParity that could be added to the
LinkedIntList class from lecture that returns whether or not the list of integers has alternating parity
(true if it does, false if not). The parity of an integer is 0 for even numbers and 1 for odd numbers. To
have alternating parity, a list would have to alternate between even and odd numbers, as in the list:
[3, 2, 19, 8, 43, 64, 1, 0, 3]
If a variable called list stores the values above, then the call of list.hasAlternatingParity()
would return true. If instead the list stored the following values, the call would return false because the
list has two even numbers in a row (4 and 12):
[2, 13, 4, 1, 0, 9, 2, 7, 4, 12, 3, 2]
By definition, an empty list or a list of one element has alternating parity. You may assume that every
element in the list is greater than or equal to 0.
Assume that we are adding this method to the LinkedIntList class as seen in lecture and as shown
below. You may not call any other methods of the class to solve this problem and your method cannot
change the contents of the list.
public class LinkedIntList {
private ListNode front;
}
methods
CSE 143 Section Handout #12
7. Recursive Tracing. For each call to the following method, indicate what output is produced:
public void mystery(int n) {
if (n % 2 == 1) {
System.out.print(n);
} else {
System.out.print(n + ", ");
mystery(n / 2);
}
}
Call
mystery(13);
mystery(42);
mystery(40);
mystery(60);
mystery(48);
Output
CSE 143 Section Handout #12
8. Recursive Programming. Write a recursive method indexOf that accepts two Strings as parameters and
that returns the starting index of the first occurrence of the second String inside the first String (or -1 if
not found). The table below lists several calls to your method and their expected return values. Notice that
case matters, as in the last example that returns -1.
Call
indexOf("Barack Obama",
indexOf("Barack Obama",
indexOf("Barack Obama",
indexOf("Barack Obama",
indexOf("Barack Obama",
"Bar")
"ck")
"a")
"McCain")
"BAR")
Value Returned
0
4
1
-1
-1
Strings have an indexOf method, but you are not allowed to call it. You are limited to these methods:
Method
equals(other)
length()
substring(fromIndex, toIndex)
substring(fromIndex)
Description
returns true if the two strings contain the same characters
returns the number of characters in the string
returns a new string containing the characters from this string
from fromIndex (inclusive) to toIndex (exclusive), or to the
end of the string if toIndex is omitted
You are not allowed to construct any structured objects other than Strings (no array, List, Scanner,
etc.) and you may not use any loops to solve this problem; you must use recursion.
CSE 143 Section Handout #12
Solution Key
1.
List
(a) [10, 20, 30]
(b) [8, 2, 9, 7, 4]
(c) [-1, 3, 28, 17, 9, 33]
Output
[31, 21, 11]
[5, 8, 10, 3, 9]
[34, 10, 18, 29, 4, 0]
2. Two solutions are shown.
public static void filterRange(ArrayList<Integer> list, int min, int max) {
for (int i = 0; i < list.size(); i++) {
if (list.get(i) >= min && list.get(i) <= max) {
list.remove(i);
i--;
}
}
}
public static void filterRange(ArrayList<Integer> list, int min, int max) {
for (int i = list.size() - 1; i >= 0; i--) {
int element = list.get(i);
if (min <= element && element <= max) {
list.remove(i);
}
}
}
3.
4.
public static int removeMin(Stack<Integer> s) {
Queue<Integer> q = new LinkedList<Integer>();
int min = s.pop();
q.add(min);
while (!s.isEmpty()) {
// s -> q, looking for min value
int next = s.pop();
if (next < min) {
min = next;
}
q.add(next);
}
while (!q.isEmpty()) {
// q -> s, filtering out occurrences of min
int next = q.remove();
if (next != min) {
s.push(next);
}
}
s2q(s, q);
// s -> q -> s, to un-reverse the order
q2s(q, s);
return min;
}
public static Set<String> whoPassed(Map<String, Integer> students,
Map<Integer, Double> grades, double desired) {
}
Set<String> result = new TreeSet<String>();
for (String name : students.keySet()) {
int grade = students.get(name);
if (grades.get(grade) >= desired) {
result.add(name);
}
}
return result;
CSE 143 Section Handout #12
Solutions (continued)
5.
6.
ListNode list2 = list.next.next.next;
list2.next = list;
list.next.next.next = list.next;
list = list.next.next;
list.next.next = null;
list2.next.next = null;
//
//
//
//
//
//
list2 -> 4
4 -> 1
3 -> 2
list -> 3
2/
1/
public boolean hasAlternatingParity() {
if (front != null) {
ListNode current = front;
while (current.next != null) {
if (current.data % 2 == current.next.data % 2) {
return false;
}
current = current.next;
}
}
return true;
}
7.
Call
mystery(13);
mystery(42);
mystery(40);
mystery(60);
mystery(48);
8.
Output
13
42,
40,
60,
48,
21
20, 10, 5
30, 15
24, 12, 6, 3
public static int indexOf(String source, String target) {
if (target.length() > source.length()) {
return -1;
} else if (source.substring(0, target.length()).equals(target)) {
return 0;
} else {
int pos = indexOf(source.substring(1), target);
if (pos == -1) {
return pos;
} else {
return pos + 1;
}
}
}