algorithm 4th笔记(1.3)
封装数据结构 Bag，Stack，Queue
- 三种数据结构各有各的特点，Bag是无序的，用来存放数据，然后找出满足条件的数据。Stack是先进后出，Queue是先进先出。
- 三种数据结构的实现分别有数组实现和链表实现。
- 使用泛型。
Bag
不支持从中删除元素的集合数据类型，它的目的就是帮助用例收集元素并迭代遍历所有收集到的元素（也可以检查背包是否为空或者获取背包中元素的数量，迭代的顺序不确定且与用例无关）
可以相像一个非常喜欢收集弹子球的人，他将所有的弹子球中寻找某种特点的弹子球。
package edu.princeton.cs.algs4;
import java.util.Iterator;
import java.util.NoSuchElementException;
public class Bag- implements Iterable
- {
private Node
- first; // beginning of bag
private int n; // number of elements in bag
// helper linked list class
private static class Node
- {
private Item item;
private Node
- next;
}
/**
* Initializes an empty bag.
*/
public Bag() {
first = null;
n = 0;
}
/**
* Returns true if this bag is empty.
*
* @return {@code true} if this bag is empty;
* {@code false} otherwise
*/
public boolean isEmpty() {
return first == null;
}
/**
* @return the number of items in this bag
*/
public int size() {
return n;
}
/**
* Adds the item to this bag.
*
* @param item the item to add to this bag
*/
public void add(Item item) {
Node
- oldfirst = first;
first = new Node
- ();
first.item = item;
first.next = oldfirst;
n++;
}
/**
* Returns an iterator that iterates over the items in this bag in arbitrary order.
*
* @return an iterator that iterates over the items in this bag in arbitrary order
*/
public Iterator
- iterator() {
return new ListIterator
- (first);
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator
- implements Iterator
- {
private Node
- current;
public ListIterator(Node
- first) {
current = first;
}
public boolean hasNext() { return current != null; }
public void remove() { throw new UnsupportedOperationException(); }
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = current.item;
current = current.next;
return item;
}
}
/**
* Unit tests the {@code Bag} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
Bag
bag = new Bag();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
bag.add(item);
}
StdOut.println("size of bag = " + bag.size());
for (String s : bag) {
StdOut.println(s);
}
}
}
Stack
先进先出（LIFO）
当你的邮件在桌子上放成一叠时，使用的就是栈。新的邮件来到时将它们放在最上面，当你有空时，你会一封一封地从上到下阅读它们
常用示例：算术表达式求值
package edu.princeton.cs.algs4;
import java.util.Iterator;
import java.util.NoSuchElementException;
public class Stack- implements Iterable
- {
private Node
- first; // top of stack
private int n; // size of the stack
// helper linked list class
private static class Node
- {
private Item item;
private Node
- next;
}
/**
* Initializes an empty stack.
*/
public Stack() {
first = null;
n = 0;
}
/**
* Returns true if this stack is empty.
*
* @return true if this stack is empty; false otherwise
*/
public boolean isEmpty() {
return first == null;
}
/**
* Returns the number of items in this stack.
*
* @return the number of items in this stack
*/
public int size() {
return n;
}
/**
* Adds the item to this stack.
*
* @param item the item to add
*/
public void push(Item item) {
Node
- oldfirst = first;
first = new Node
- ();
first.item = item;
first.next = oldfirst;
n++;
}
/**
* Removes and returns the item most recently added to this stack.
*
* @return the item most recently added
* @throws NoSuchElementException if this stack is empty
*/
public Item pop() {
if (isEmpty()) throw new NoSuchElementException("Stack underflow");
Item item = first.item; // save item to return
first = first.next; // delete first node
n--;
return item; // return the saved item
}
/**
* Returns (but does not remove) the item most recently added to this stack.
*
* @return the item most recently added to this stack
* @throws NoSuchElementException if this stack is empty
*/
public Item peek() {
if (isEmpty()) throw new NoSuchElementException("Stack underflow");
return first.item;
}
/**
* Returns a string representation of this stack.
*
* @return the sequence of items in this stack in LIFO order, separated by spaces
*/
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this) {
s.append(item);
s.append(' ');
}
return s.toString();
}
/**
* Returns an iterator to this stack that iterates through the items in LIFO order.
*
* @return an iterator to this stack that iterates through the items in LIFO order
*/
public Iterator
- iterator() {
return new ListIterator
- (first);
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator
- implements Iterator
- {
private Node
- current;
public ListIterator(Node
- first) {
current = first;
}
public boolean hasNext() {
return current != null;
}
public void remove() {
throw new UnsupportedOperationException();
}
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = current.item;
current = current.next;
return item;
}
}
/**
* Unit tests the {@code Stack} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
Stack
stack = new Stack();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
if (!item.equals("-"))
stack.push(item);
else if (!stack.isEmpty())
StdOut.print(stack.pop() + " ");
}
StdOut.println("(" + stack.size() + " left on stack)");
}
}
Queue
package edu.princeton.cs.algs4;
import java.util.Iterator;
import java.util.NoSuchElementException;
public class Queue- implements Iterable
- {
private Node
- first; // beginning of queue
private Node
- last; // end of queue
private int n; // number of elements on queue
// helper linked list class
private static class Node
- {
private Item item;
private Node
- next;
}
/**
* Initializes an empty queue.
*/
public Queue() {
first = null;
last = null;
n = 0;
}
/**
* Returns true if this queue is empty.
*
* @return {@code true} if this queue is empty; {@code false} otherwise
*/
public boolean isEmpty() {
return first == null;
}
/**
* Returns the number of items in this queue.
*
* @return the number of items in this queue
*/
public int size() {
return n;
}
/**
* Returns the item least recently added to this queue.
*
* @return the item least recently added to this queue
* @throws NoSuchElementException if this queue is empty
*/
public Item peek() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
return first.item;
}
/**
* Adds the item to this queue.
*
* @param item the item to add
*/
public void enqueue(Item item) {
Node
- oldlast = last;
last = new Node
- ();
last.item = item;
last.next = null;
if (isEmpty()) first = last;
else oldlast.next = last;
n++;
}
/**
* Removes and returns the item on this queue that was least recently added.
*
* @return the item on this queue that was least recently added
* @throws NoSuchElementException if this queue is empty
*/
public Item dequeue() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
Item item = first.item;
first = first.next;
n--;
if (isEmpty()) last = null; // to avoid loitering
return item;
}
/**
* Returns a string representation of this queue.
*
* @return the sequence of items in FIFO order, separated by spaces
*/
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this) {
s.append(item);
s.append(' ');
}
return s.toString();
}
/**
* Returns an iterator that iterates over the items in this queue in FIFO order.
*
* @return an iterator that iterates over the items in this queue in FIFO order
*/
public Iterator
- iterator() {
return new ListIterator
- (first);
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator
- implements Iterator
- {
private Node
- current;
public ListIterator(Node
- first) {
current = first;
}
public boolean hasNext() { return current != null; }
public void remove() { throw new UnsupportedOperationException(); }
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = current.item;
current = current.next;
return item;
}
}
/**
* Unit tests the {@code Queue} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
Queue
queue = new Queue();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
if (!item.equals("-"))
queue.enqueue(item);
else if (!queue.isEmpty())
StdOut.print(queue.dequeue() + " ");
}
StdOut.println("(" + queue.size() + " left on queue)");
}
}