2018 cs61b dis mid
https://sp18.datastructur.es/materials/exam/cs61b-sp18-mt1.pdf
1.Static Data
a.
public static void main(String[] args) {
String a = "alice";
String b = "bob";
String c = "carol";
String d = "dan";
String[][] twod = {{a, b}, {c, d}};
String[] X = twod[1];
printStringArray(X); //carol dan
Dada.rs = X;
String[] Y = Dada.rs;
Y = new String[]{d, c};
Dada.rs[1] = "eve";
printStringArray(Dada.rs); //carol eve
printStringArray(Y); //dan carol
printStringArray(twod[0]); //alice bob
printStringArray(twod[1]); //carol eve
}
b.
public static void main(String[] args) {
String a = "alice";
String b = "bob";
String c = "carol";
String d = "dan";
String[][] twod = {{a, b}, {c, d}};
Dada.rs = new String[]{"fritz", "gritz"};
String[] X = twod[0];
printStringArray(X); //alice bob
fillOne(X[0]);//formal parameter
printStringArray(X); // alice bob
fillMany(X);
printStringArray(X); //fritz gritz
}
2What It Do
a.
eg:这是前二十个
对于 x>=1 这个是输出小于x的2的次方数,小于1就是无限循环函数。
b.这个题目意思画出图来表示列表的指向,如果对象没有被引用（就是与其他无关）称作garbage collected
3.KeyGate
从sb.unlock(k)看作第一个那么
1.UK
2.UK
3.UK
4.UK//sk继承于k
5.UK//sk继承于k
6.UF
7.Compile Error
8.Compile Error
9.UF
10.blank
11.UK//sk继承于k
12.UK//sk继承于k
13.Runtime Error//父类不能转换为子类
4.Sans
a.
public static int[] sans(int[] x, int y) {
int[] xclean = new int[x.length];
int c = 0;
for (int i = 0; i < x.length; i += 1) {
if (x[i] != y) {
xclean[c] = x[i];
c += 1;
}
}
int[] r = new int[c];
System.arraycopy(xclean, 0, r, 0, c);
return r;
}
b.
public static IntList ilsans(IntList x, int y) {
if (x == null) {
return null;
}
if (x.first == y) {
return ilsans(x.rest, y);
}
return new IntList(x.first, ilsans(x.rest, y));
}
c.
public static IntList ilsans(IntList x, int y) {
if (x == null) {
return null;
}
x.rest = ilsans(x.rest, y);
if (x.first == y) {
return x.rest;
}
return x;
}
d.
public void testSans() { // TEST THE ARRAY VERSION OF SANS
int[] x = new IntList.of(4, 5, 6);
int y = 5
int[] expected = new IntList.of(4, 6);
int[] actual = sans(x, y);
assertArrayEquals(expected, actual);
}
public void testIlsans() {
IntList x = IntList.of(1, 2, 3);
int y = 2;
IntList expected = IntList.of(1, 3);
IntList actual = Sans.ilsans(x, y);
assertEquals(expected, actual);
assertNotEquals(x, actual); // x should not have been altered
}
a.
public class ArrayStack<Item> implements Stack<Item> {
private Item[] items;
private int size;
public ArrayStack() { // initial array size is 8
items = (Item[]) new Object[8];
size = 0;
}
private void resize(int capacity) { // resizes array to given capacity
Item[] temp = (Item[]) new Object[capacity];
for (int i = 0 ;i < size; i++){
temp[i] = items[i];
}
items = temp;
}
public void push(Item x) {
if (usageRatio() == 1) {
resize(size * 2);
}
items[size] = x;
size++;
}
public Item pop() { // returns null if stack is empty
if (size == 0) { return null; }
if (usageRatio() < 0.25 && items.length > 8) { resize(items.length / 2); }
size = size - 1;
Item returnItem = items[size];
items[size] = null;
return returnItem;
}
public int size() { return size; }
private double usageRatio() { return ((double) size()) / items.length; }
}
b.
先全部pop出来，然后如果和x不相等就push进去
public default void purge(Item x) {
if (size() == 0) {
return;
}
Item top = pop();
purge(x);
if (!x.equals(top)) {
push(top);
}
}
6.Combine
a.
public class Combine {
public static int combine(ComFunc f, int[] x) {
if (x.length == 0) {
return 0;
}
if (x.length == 1) {
return x[0];
}
int t = f.apply(x[0], x[1]);
return help(f, x, t, 2);
}
private static int help(ComFunc f, int[] x, int t, int k) {
if (k == x.length) {
return t;
}
t = f.apply(x[k], t);
return help(f, x, t, k + 1);
}
b.
public static int sumAll(int[] x) { // sumAll is not a member of Combine
return combine(new Add(), x);
}
7.The Downside of Default
a.
default public void plusEquals(ListOfInts x) { // assume x is non-null
if (x.size() != this.size()){ return; }
for (int i = 0; i < this.length; i++) {
this.set(i, x.get(i) + this.get(i));
}
}
b.
public class DLListOfInts implements ListOfInts {
public class IntNode {
public int item;
public IntNode next, prev;
}
public IntNode sentinel;
public int size;
@Override
public void plusEquals(DLListOfInts x) {
if (x.size() != this.size()){ return; }
IntNode xp = x.sentinel.next;
for (IntNode p = sentinel.next; p != sentinel; p = p.next) {
p.item = xp.item + p.item;
xp = xp.next;
}
}
c.
1.返回类型不一致，result是ListOfInts而函数类型是DLLisOfInts
2.破坏了原数组
