我很难理解为什么我无法从堆栈中弹出任何值，并且想知道这里是否有人可以帮助我。问题在于pop方法，但我不知道它有什么问题。
import java.io.File;
import java.io.IOException;
import java.util.ListIterator;
import java.util.Random;
public class LinkedListStack {
DoublyLinkedList<Integer> list = new DoublyLinkedList<Integer>();
public int size() {
return list.size();
}
public void push(Integer s) {
list.add(s);
}
public Integer pop() {
list.iterator().previous();
Integer i = list.iterator().next();
list.iterator().remove();
return i;
}
public Integer top() {
return list.iterator().next();
}
public boolean isEmpty() {
return list.isEmpty();
}
public String displayStack() {
return (list.toString());
}
public static void main(String[] args) throws IOException {
LinkedListStack stack = new LinkedListStack();
int n, seed;
File outputFile;
File dir = new File(".");
if (args.length > 0) {
n = Integer.parseInt(args[0]);
seed = Integer.parseInt(args[1]);
}else {
n = 10;
seed = 1;
}
if (args.length == 3) {
outputFile = new File(args[2]);
} else {
outputFile = new File(dir.getCanonicalPath() + File.separator + "Files/testOut_Stack");
}
OutputWriter out = new OutputWriter(outputFile);
Random r = new Random(seed);
Integer nextval;
for (int i = 0; i < n; i++) {
nextval = r.nextInt(10000);
if (stack.isEmpty()) {
stack.push(nextval);
}
if (nextval > stack.top()) {
stack.pop();
stack.push(nextval);
}
/*if (i == 0) {
stack.push(nextval);
} else if (nextval > stack.top()) {
stack.pop();
stack.push(nextval);
}*/
/* retain the max value that you see among the integers generated in the stack.
* In the end there should be only one integer in stack, which is the max value
*/
}
// write the content of stack -- which is the max value -- to file
out.writeOutput(stack.displayStack());
}
}
双重链接列表类
import java.io.File;
import java.io.IOException;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Random;
import csci3230.hw3.OutputWriter;
public class DoublyLinkedList<Item> implements Iterable<Item> {
private int n; // number of elements on list
private Node pre; // sentinel before first item
private Node post; // sentinel after last item
public DoublyLinkedList() {
pre = new Node();
post = new Node();
pre.next = post;
post.prev = pre;
}
// linked list node helper data type
private class Node {
private Item item;
private Node next;
private Node prev;
}
public boolean isEmpty() {
return (n == 0);
// your code
}
public int size() {
return n;
// your code
}
// add the item to the list
public void add(Item item) {
Node last = post.prev;
Node x = new Node();
x.item = item;
x.next = post;
x.prev = last;
post.prev = x;
last.next = x;
n++;
// your code
}
public ListIterator<Item> iterator() { return new DoublyLinkedListIterator(); }
// assumes no calls to DoublyLinkedList.add() during iteration
private class DoublyLinkedListIterator implements ListIterator<Item> {
private Node current = pre.next; // the node that is returned by next()
private Node lastAccessed = null; // the last node to be returned by prev() or next()
// reset to null upon intervening remove() or add()
private int index = 0;
public boolean hasNext() {
return (index < n); // your code
}
public boolean hasPrevious() {
return (index > 0);
// your code
}
public int previousIndex() {
return (index - 1);
// your code
}
public int nextIndex() {
return index;
// your code
}
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
lastAccessed = current;
Item item = current.item;
current = current.next;
index++;
return item;
// your code
}
public Item previous() {
if (!hasPrevious()) throw new NoSuchElementException();
current = current.prev;
index--;
lastAccessed = current;
return current.item;
// your code
}
// replace the item of the element that was last accessed by next() or previous()
// condition: no calls to remove() or add() after last call to next() or previous()
public void set(Item item) {
if (lastAccessed == null) throw new IllegalStateException();
lastAccessed.item = item;
// your code
}
// remove the element that was last accessed by next() or previous()
// condition: no calls to remove() or add() after last call to next() or previous()
public void remove() {
if (lastAccessed == null) throw new IllegalStateException();
Node x = lastAccessed.prev;
Node y = lastAccessed.next;
x.next = y;
y.prev = x;
n--;
if (current == lastAccessed)
current = y;
else
index--;
lastAccessed = null;
// your code
}
// add element to list
public void add(Item item) {
Node x = current.prev;
Node y = new Node();
Node z = current;
y.item = item;
x.next = y;
y.next = z;
z.prev = y;
y.prev = x;
n++;
index++;
lastAccessed = null;
// your code
}
}
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this)
s.append(item + " ");
return s.toString();
}
/* a test client
* arg[1]: number of input values
* arg[2]: random seed
* arg[3]: output file path
*/
public static void main(String[] args) throws IOException {
int n, seed;
File outputFile;
File dir = new File(".");
if (args.length > 0) {
n = Integer.parseInt(args[0]);
seed = Integer.parseInt(args[1]);
}else {
n = 10;
seed = 1;
}
if (args.length == 3) {
outputFile = new File(args[2]);
} else {
outputFile = new File(dir.getCanonicalPath() + File.separator + "Files/testOut_DLL");
}
OutputWriter out = new OutputWriter(outputFile);
Random r = new Random(seed);
// add elements 1, ..., n
out.writeOutput("Integer items to insert: "+ n);
DoublyLinkedList<Integer> list = new DoublyLinkedList<Integer>();
for (int i = 0; i < n; i++)
list.add(r.nextInt(100));
out.writeOutput(list.toString());
ListIterator<Integer> iterator = list.iterator();
// go forwards with next() and set()
out.writeOutput("add 1 to each element via next() and set()");
while (iterator.hasNext()) {
int x = iterator.next();
iterator.set(x + 1);
}
out.writeOutput(list.toString());
// go backwards with previous() and set()
out.writeOutput("multiply each element by 3 via previous() and set()");
while (iterator.hasPrevious()) {
int x = iterator.previous();
iterator.set(x + x + x);
}
out.writeOutput(list.toString());
// remove all elements that are multiples of 4 via next() and remove()
out.writeOutput("remove elements that are a multiple of 4 via next() and remove()");
while (iterator.hasNext()) {
int x = iterator.next();
if (x % 4 == 0) iterator.remove();
}
out.writeOutput(list.toString());
// remove all even elements via previous() and remove()
out.writeOutput("remove elements that are even via previous() and remove()");
while (iterator.hasPrevious()) {
int x = iterator.previous();
if (x % 2 == 0) iterator.remove();
}
out.writeOutput(list.toString());
// add elements via next() and add()
out.writeOutput("add elements via next() and add()");
while (iterator.hasNext()) {
int x = iterator.next();
iterator.add(x + 1);
}
out.writeOutput(list.toString());
// add elements via previous() and add()
out.writeOutput("add elements via previous() and add()");
while (iterator.hasPrevious()) {
int x = iterator.previous();
iterator.add(x * 10);
iterator.previous();
}
out.writeOutput(list.toString());
}
}