题目正文：
http://coursera.cs.princeton.edu/algs4/assignments/queues.html
作业难点：
1、选择使用链表还是数组，将会是第一个问题，选择合适会减少很多工作量。
2、并行从RandomizedQueue中取两个枚举，这两个枚举必须是不一样的，所以有很多偷懒的“伪随机”是行不通的。
3、SubSet仅需K存储而不是N存储，即可实现。（这个我也没想到方法实现）
作业技巧：
1、可遍数组和邻接节点两种数据结构都已经给你了，你只要改改，基本上实现第一个Deque没有任何难度
2、RandomizedQueue涉及到一个基本面试题问题，如果有一个0到100的数组，随机取不重复的元素，如何实现？
从大小为N的数组A中随机[0,N)取下标x的元素A[x]，让A[x]与A[N-1]交换，再随机[0,N-1)取下标x的元素A[y]与A[N-2]交换，以此类推。
3、无论是编译还是网站测试都会报警数组泛型的问题，这个可以百度，JAVA数组不支持泛型。对于作业来说忽略就好了。
代码参考：
（这是我自己亲测100分的答案，不代表写得最好，请在自己实在完成不了的时候再看，不然的话做这个题目的意义一点都没有）
1 import edu.princeton.cs.algs4.StdIn;
2 import edu.princeton.cs.algs4.StdOut;
3
4 import java.util.Iterator;
5 import java.util.NoSuchElementException;
6
7
8 public class Deque<Item> implements Iterable<Item> {
9 private int n;
10 private Node first;
11 private Node last;
12
13 public Deque() // construct an empty deque
14 {
15 n = 0;
16 first = null;
17 last = null;
18 }
19
20 public boolean isEmpty() // is the deque empty?
21 {
22 return first == null;
23 }
24
25 public int size() // return the number of items on the deque
26 {
27 return n;
28 }
29
30 public void addFirst(Item item) // add the item to the front
31 {
32 if (item == null) {
33 throw new NullPointerException();
34 }
35
36 Node oldfirst = first;
37 first = new Node();
38 first.item = item;
39 first.next = oldfirst;
40
41 if (oldfirst == null) {
42 last = first;
43 } else {
44 oldfirst.prev = first;
45 }
46
47 n++;
48 }
49
50 public void addLast(Item item) // add the item to the end
51 {
52 if (item == null) {
53 throw new NullPointerException();
54 }
55
56 Node oldlast = last;
57 last = new Node();
58 last.item = item;
59 last.prev = oldlast;
60
61 if (oldlast == null) {
62 first = last;
63 } else {
64 oldlast.next = last;
65 }
66
67 n++;
68 }
69
70 public Item removeFirst() // remove and return the item from the front
71 {
72 if (isEmpty()) {
73 throw new NoSuchElementException();
74 }
75
76 Item item = first.item;
77 first = first.next;
78
79 if (first == null) {
80 last = null;
81 } else {
82 first.prev = null;
83 }
84
85 n--;
86
87 return item;
88 }
89
90 public Item removeLast() // remove and return the item from the end
91 {
92 if (isEmpty()) {
93 throw new NoSuchElementException();
94 }
95
96 Item item = last.item;
97 last = last.prev;
98
99 if (last == null) {
100 first = null;
101 } else {
102 last.next = null;
103 }
104
105 n--;
106
107 return item;
108 }
109
110 public Iterator<Item> iterator() // return an iterator over items in order from front to end
111 {
112 return new DequeIterator();
113 }
114
115 public static void main(String[] args) // unit testing
116 {
117 Deque<String> deque = new Deque<String>();
118
119 while (!StdIn.isEmpty()) {
120 String item = StdIn.readString();
121
122 if (item.equals("+")) {
123 item = StdIn.readString();
124 deque.addFirst(item);
125 } else if (item.equals("++")) {
126 item = StdIn.readString();
127 deque.addLast(item);
128 } else if (item.equals("exit")) {
129 break;
130 } else if (item.equals("show")) {
131 for (String s : deque) {
132 StdOut.println(s);
133 }
134 } else if (item.equals("-")) {
135 if (!deque.isEmpty()) {
136 StdOut.print(deque.removeFirst() + " ");
137 }
138 } else if (item.equals("--")) {
139 if (!deque.isEmpty()) {
140 StdOut.print(deque.removeLast() + " ");
141 }
142 }
143 }
144
145 StdOut.println("(" + deque.size() + " left on deque)");
146 }
147
148 private class DequeIterator implements Iterator<Item> {
149 private Node current = first;
150
151 public boolean hasNext() {
152 return current != null;
153 }
154
155 public void remove() {
156 throw new UnsupportedOperationException();
157 }
158
159 public Item next() {
160 if (!hasNext()) {
161 throw new NoSuchElementException();
162 }
163
164 Item item = current.item;
165 current = current.next;
166
167 return item;
168 }
169 }
170
171 private class Node {
172 private Item item;
173 private Node next;
174 private Node prev;
175 }
176 }
Deque
1 import edu.princeton.cs.algs4.StdIn;
2 import edu.princeton.cs.algs4.StdOut;
3 import edu.princeton.cs.algs4.StdRandom;
4
5 import java.util.Iterator;
6 import java.util.NoSuchElementException;
7
8
9 public class RandomizedQueue<Item> implements Iterable<Item> {
10 private Item[] a; // array of items
11 private int n; // number of elements on stack
12
13 public RandomizedQueue() // construct an empty randomized queue
14 {
15 a = (Item[]) new Object[2];
16 n = 0;
17 }
18
19 public boolean isEmpty() // is the queue empty?
20 {
21 return n == 0;
22 }
23
24 public int size() // return the number of items on the queue
25 {
26 return n;
27 }
28
29 // resize the underlying array holding the elements
30 private void resize(int capacity) {
31 assert capacity >= n;
32
33 Item[] temp = (Item[]) new Object[capacity];
34
35 for (int i = 0; i < n; i++) {
36 temp[i] = a[i];
37 }
38
39 a = temp;
40 }
41
42 public void enqueue(Item item) // add the item
43 {
44 if (item == null) {
45 throw new NullPointerException();
46 }
47
48 if (n == a.length) {
49 resize(2 * a.length); // double size of array if necessary
50 }
51
52 a[n++] = item; // add item
53 }
54
55 public Item dequeue() // remove and return a random item
56 {
57 if (isEmpty()) {
58 throw new NoSuchElementException();
59 }
60
61 int targetid = StdRandom.uniform(0, n);
62 Item item = a[targetid];
63 a[targetid] = a[n - 1];
64 a[n - 1] = null; // to avoid loitering
65 n--;
66
67 // shrink size of array if necessary
68 if ((n > 0) && (n == (a.length / 4))) {
69 resize(a.length / 2);
70 }
71
72 return item;
73 }
74
75 public Item sample() // return (but do not remove) a random item
76 {
77 if (isEmpty()) {
78 throw new NoSuchElementException();
79 }
80
81 int targetid = StdRandom.uniform(0, n);
82
83 return a[targetid];
84 }
85
86 public Iterator<Item> iterator() // return an independent iterator over items in random order
87 {
88 return new RandomizedQueueIterator();
89 }
90
91 public static void main(String[] args) // unit testing
92 {
93 RandomizedQueue<String> queue = new RandomizedQueue<String>();
94
95 while (!StdIn.isEmpty()) {
96 String item = StdIn.readString();
97
98 if (item.equals("-") && !queue.isEmpty()) {
99 StdOut.print(queue.dequeue() + " ");
100 }
101 else if (item.equals("--") && !queue.isEmpty()) {
102 StdOut.print(queue.sample() + " ");
103 }
104 else if (item.equals("show")) {
105 for (String s : queue) {
106 StdOut.println(s);
107 }
108 }
109 else if (item.equals("size")) {
110 StdOut.println("(" + queue.size() + " left on stack)");
111 }
112 else if (item.equals("exit")) {
113 break;
114 }
115 else {
116 queue.enqueue(item);
117 }
118 }
119
120 StdOut.println("(" + queue.size() + " left on stack)");
121 }
122
123 private class RandomizedQueueIterator implements Iterator<Item> {
124 private int i;
125 private Item[] temp;
126
127 public RandomizedQueueIterator() {
128 i = n - 1;
129 temp = (Item[]) new Object[a.length];
130 for (int j = 0; j < n; j++) {
131 temp[j] = a[j];
132 }
133 }
134
135 public boolean hasNext() {
136 return i >= 0;
137 }
138
139 public void remove() {
140 throw new UnsupportedOperationException();
141 }
142
143 public Item next() {
144 if (!hasNext()) {
145 throw new NoSuchElementException();
146 }
147
148 int targetid = StdRandom.uniform(0, i + 1);
149 Item item = temp[targetid];
150 temp[targetid] = temp[i];
151 temp[i] = item;
152 i--;
153 return item;
154 }
155 }
156 }
RandomizedQueue
1 import edu.princeton.cs.algs4.StdIn;
2 import edu.princeton.cs.algs4.StdOut;
3 public class Subset {
4 public static void main(String[] args) {
5 RandomizedQueue<String> queue = new RandomizedQueue<String>();
6 int num = Integer.parseInt(args[0]);
7
8 while (!StdIn.isEmpty()) {
9 String item = StdIn.readString();
10 // if (item.equals("exit")) {
11 // break;
12 // }
13 queue.enqueue(item);
14 }
15
16 for (String s : queue) {
17 if (num == 0) {
18 break;
19 }
20
21 StdOut.println(s);
22 num--;
23 }
24 }
25 }
Subset
时间： 2024-10-19 07:16:04