【发布时间】：2011-02-21 19:32:53
【问题描述】：
我在使用循环数组实现这个双端队列时遇到了很多麻烦；特别是，无论我尝试什么，删除方法似乎都在删除错误的元素。有人可以帮忙吗？
public class ArrayDeque
{
public static final int INIT_CAPACITY = 8; // initial array capacity
protected int capacity; // current capacity of the array
protected int front; // index of the front element
protected int rear; // index of the rear element
protected int[] A; // array deque
public ArrayDeque( ) // constructor method
{
A = new int[ INIT_CAPACITY ];
capacity = INIT_CAPACITY;
front = rear = 0;
}
/**
* Returns the number of items in this collection.
* @return the number of items in this collection.
*/
public int size( )
{
return rear - front;
}
/**
* Returns true if this collection is empty.
* @return true if this collection is empty.
*/
public boolean isEmpty( )
{
return front == rear;
}
/**
* Returns the first element of the deque
*/
public int getFirst() throws EmptyDequeException
{
if(isEmpty()){
throw new EmptyDequeException("Deque is empty.");
}
return A[front % capacity];
}
/**
* Returns the last element of the deque
*/
public int getLast( ) throws EmptyDequeException
{
if(isEmpty()){
throw new EmptyDequeException("Deque is empty.");
}
return A[(front + rear - 1) % capacity]; // replace this line with your code
}
/**
* Inserts e at the beginning (as the first element) of the deque
*/
public void insertFirst( int e )
{
rear++;
if(size() == capacity){
capacity *= 2;
}
int[] B = new int[capacity];
for(int i = 0; i < size(); i++){
B[i] = A[i];
}
A = B;
for(int i = size(); i >= front; i--){
A[i+1] = A[i];
}
A[front] = e;
front = front % capacity;
}
/**
* Inserts e at the end (as the last element) of the deque
*/
public void insertLast( int e )
{
if(size() == capacity){
capacity *= 2;
A[rear++] = e;
}
else{
A[rear++] = e;
}
rear++;
}
/**
* Removes and returns the first element of the deque
* Shrink array by half of current size N when number of elements in the deque falls below N/4
* minimum capacity should always be 8
*/
public int removeFirst( ) throws EmptyDequeException
{
if(size() == 0){
throw new EmptyDequeException("Deque is empty.");
}
if(capacity >= 8){
if(size() < capacity/4){
capacity /= 2;
}
}
int[] B = new int[capacity];
for(int i = 1; i < size(); i++){
B[i-1] = A[i];
}
A = B;
return A[front];
}
/**
* Removes and returns the last element of the deque
* Shrink array by half of current size N when number of elements in the deque falls below N/4
* minimum capacity should always be 8
*/
public int removeLast( ) throws EmptyDequeException
{
if(size() == 0){
throw new EmptyDequeException("Deque is empty.");
}
if(capacity >= 8){
if(size() < capacity/4){
capacity /= 2;
}
}
int[] B = new int[capacity];
for(int i = front; i<size()-1; i++){
B[i] = A[i];
}
A = B;
rear--;
return A[rear];
}
} // end class
【问题讨论】：
-
我一直觉得对这类事情很有帮助的是编写一个帮助方法来打印出数组的内容，这将使您可以确保所有其他功能首先正常工作。
-
那不是循环数组，那是普通数组。循环数组索引环绕。更正式地说，
A[i] == A[i%A.length]表示所有整数i。在上述的圆形阵列实现中，后部有时会小于前部。此外，只有在容量发生变化时，您才需要创建一个新数组。 -
正如 outis 指出的那样，
front和rear应始终在[0, capacity-1]范围内。在您的代码中，有时您使用%对其进行维护，但其他时候您假设front可以是>= capacity。选择一种或另一种方式并坚持下去。此外，您忘记调整getLast()告诉您放置代码的位置。 ;)
标签： java deque circular-buffer