612.1.003 ALGS4 | Stacks and Queues

Algorithm | Coursera - by Robert Sedgewick

Type the code one by one!

不要拜读——只写最有感触的！而不是仅仅做一个笔记摘录员，那样毫无意义！

动手实现。看懂不等于会写——用sublime 写，而非ide（强迫白板写代码）—— 默写代码

直接看ppt优先， 而不是被动看视频

不要拜读——没必要看废话

循循善诱——要是我大一时候看这本书就好了！但是现在开始也不算晚.

很多知识不用查，自己拿编译器print查看

时刻记得资源有限——这样才能写出能够应对大数据时代的高效率代码

要学会写完整能跑起来的程序，及时反馈，有成就感 —— 多巴胺

Inspiration

1.link-list vs resizing array

• 确保每个操作都能很快完成 —— 链表实现

• 如果只关心总的实现实现 —— 数组实现

2.errors - Guiding Principles - use generics

• Welcome compile-time errors; //有问题在编译时提前发现
• Avoid run-time errors. // 确保在交给客户时避免runtime error

3.优秀的代码不该使用强制类型转换

若类型不匹配，容易发生runtime error

4.Iterator

Java supports elegant client code

What I can use - Code

Stack（Generic +iteration）

/******************************************************************************
 *  Compilation:  javac Stack.java
 *  Execution:    java Stack < input.txt
 *  Dependencies: StdIn.java StdOut.java
 *  Data files:   https://algs4.cs.princeton.edu/13stacks/tobe.txt
 *
 *  A generic stack, implemented using a singly linked list.
 *  Each stack element is of type Item.
 *
 *  This version uses a static nested class Node (to save 8 bytes per
 *  Node), whereas the version in the textbook uses a non-static nested
 *  class (for simplicity).
 *
 *  % more tobe.txt
 *  to be or not to - be - - that - - - is
 *
 *  % java Stack < tobe.txt
 *  to be not that or be (2 left on stack)
 *
 ******************************************************************************/

//package edu.princeton.cs.algs4;

import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 *  The {@code Stack} class represents a last-in-first-out (LIFO) stack of generic items.
 *  It supports the usual <em>push</em> and <em>pop</em> operations, along with methods
 *  for peeking at the top item, testing if the stack is empty, and iterating through
 *  the items in LIFO order.
 *  <p>
 *  This implementation uses a singly linked list with a static nested class for
 *  linked-list nodes. See {@link LinkedStack} for the version from the
 *  textbook that uses a non-static nested class.
 *  See {@link ResizingArrayStack} for a version that uses a resizing array.
 *  The <em>push</em>, <em>pop</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em>
 *  operations all take constant time in the worst case.
 *  <p>
 *  For additional documentation,
 *  see <a href="https://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 *
 *  @param <Item> the generic type of an item in this stack
 */
public class Stack<Item> implements Iterable<Item> {
    private Node<Item> first;     // top of stack
    private int n;                // size of the stack

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> 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<Item> oldfirst = first;
        first = new Node<Item>();
        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<Item> iterator() {
        return new ListIterator<Item>(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> 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<String> stack = new Stack<String>();
        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)");
    }
}

/******************************************************************************
 *  Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
 *
 *  This file is part of algs4.jar, which accompanies the textbook
 *
 *      Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
 *      Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
 *      http://algs4.cs.princeton.edu
 *
 *
 *  algs4.jar is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  algs4.jar is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with algs4.jar.  If not, see http://www.gnu.org/licenses.
 ******************************************************************************/

Queue（Generic +iteration）

/******************************************************************************
 *  Compilation:  javac Queue.java
 *  Execution:    java Queue < input.txt
 *  Dependencies: StdIn.java StdOut.java
 *  Data files:   https://algs4.cs.princeton.edu/13stacks/tobe.txt
 *
 *  A generic queue, implemented using a linked list.
 *
 *  % java Queue < tobe.txt
 *  to be or not to be (2 left on queue)
 *
 ******************************************************************************/

//package edu.princeton.cs.algs4;
import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 *  The {@code Queue} class represents a first-in-first-out (FIFO)
 *  queue of generic items.
 *  It supports the usual <em>enqueue</em> and <em>dequeue</em>
 *  operations, along with methods for peeking at the first item,
 *  testing if the queue is empty, and iterating through
 *  the items in FIFO order.
 *  <p>
 *  This implementation uses a singly linked list with a static nested class for
 *  linked-list nodes. See {@link LinkedQueue} for the version from the
 *  textbook that uses a non-static nested class.
 *  See {@link ResizingArrayQueue} for a version that uses a resizing array.
 *  The <em>enqueue</em>, <em>dequeue</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em>
 *  operations all take constant time in the worst case.
 *  <p>
 *  For additional documentation, see <a href="https://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 *
 *  @param <Item> the generic type of an item in this queue
 */
public class Queue<Item> implements Iterable<Item> {
    private Node<Item> first;    // beginning of queue
    private Node<Item> last;     // end of queue
    private int n;               // number of elements on queue

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> 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<Item> oldlast = last;
        last = new Node<Item>();
        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<Item> iterator()  {
        return new ListIterator<Item>(first);
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> 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<String> queue = new Queue<String>();
        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)");
    }
}

/******************************************************************************
 *  Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
 *
 *  This file is part of algs4.jar, which accompanies the textbook
 *
 *      Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
 *      Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
 *      http://algs4.cs.princeton.edu
 *
 *
 *  algs4.jar is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  algs4.jar is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with algs4.jar.  If not, see http://www.gnu.org/licenses.
 ******************************************************************************/

Queues,and Stacks

• stacks
• resizing arrays
• queues
• generics
• iterators
• applications

1 Stack

Stack1：linked-list implementation

链表在进行循环遍历时效率不高，但是插入和删除时优势明显。

用链表实现栈

Pop Operation

Push Operation

public class LinkedStackOfStrings
{
    private Node first = null;

    //inner class
    private class Node
    {
        String item;
        Node next;
    }
    public boolean isEmpty()
    {
        return first == null;
    }
    private String pop()
    {
        String item = first.item;//pop the first item
        first = first.next;
        return first;
    }

    public void push(String item){
        Node oldfirst = first;
        first = new Node();
        first.item = item;//push the new item
        first.next = oldfirst;
    }
}

Stack2：array implementation

Solve Defect —— use resizing array

lower running time

• Considerations

the art of coding

• Loitering 对象游离
  
  在栈的数组实现中有对象的引用，而我们没有真正使用它。
  
  最好将去除的元素对应的项设为空。
  
  否则只是没有引用指向它，但是那个位置的内存仍然被占用着
  
  

public class ResizingArrayStackOfStrings()
{
	private String[] s;
	private int N = 0;

	// public FixedCapacityStackOfStrings(int capacity)
	// {
	// 	s = new String[capacity];//constructor;fixedcappacity
	// }

	public ResizingArrayStackOfStrings()//constructor
	{
		s = new String[1];
	}

	public boolean isEmpty()
	{
		return N == 0;
	}
	//Array is between 25% and 100% full.
	private void resize(int capacity)
	{
		String[] copy = new String[capacity];
		for (int i = 0;i < N; i++)
			copy[i] = s[i];
		s = copy; // assign copy to s
	}

	//Q. How to grow array?
	//A. If array is full, create a new array of twice the size, and copy items
	//Consequence:Inserting first N items takes time proportional to N (not N^2 ).
	public void push(String item)
	{
		if (N == s.length) resize(2 *  s.length);//If array is full, create a new array of twice the size, and copy items.
		S[N++] = item; //use the index into array,then increment N // brilliant!

	}

	//	Q. How to shrink array?
	//halve size of array s[] when array is one-quarter full.
	//当数组1/4满的时候再将容量减半——防止thrashing 在临界值抖动 不断扩数组和缩数组造成浪费
	public String pop()
	{
		String item = s[--N];//decrement N ;then use to index into array
		s[N] = null;//to solve the Loitering problem. assign null to wasted space
		if (N > 0 && N == s.length/4){
			resize(s.length/2);
		}
		return item;

	}

}

Tradeoffs:resizing array vs. linked list

• 确保每个操作都能很快完成 —— 链表实现
• 如果只关心总的实现实现 —— 数组实现

2 Queue

Queue1：linked-list implementation

dequeue

enqueue

public class LinkedQueueOfStrings
{
	private Node first,last;

	private class Node
	{
		String item;
		Node next;
	}

	public boolean isEmpty()
	{
		return first == null;
	}

	public String enqueue(String item)
	{
		Node oldlast = last;
		last = new Node();
		last.item = item;
		last.next = null;
		if(isEmpty()) first = last;//special cases for empty queue
		else 	oldlast.next = last;
	}

	public String dequeue()
	{
		String item = first.item;
		firt = first.next;
		if(isEmpty()) last = null;//special cases for empty queue
		return item;
	}
}

Queue2：resizing array implementation

public class ResizingArrayQueue<Item> implements Iterable<Item> //generic type name

{
	private Item[] q;  // queue elements

	public ResizingArrayQueue(){
		q = (Item[]) new Object[2];
		n = 0;//the real number of items in this queue.
		first = 0;
		last = 0;
	}

	public boolean isEmpty()
	{
		return n == 0;
	}

	public int size(){
		return n;//Returns the number of items in this queue.
	}

	private void resize(int capacity){
		assert capacity >=n;
		Item[] temp = (Item[]) new Object[capacity];
		for (int i = 0; i < n; i++){
			temp[i] = q[(first+i)%q.length];//q.length (empty array included)
		}
		q = temp;
		first = 0;
		last = n;
	}

	public String enqueue(Item item)
	{
		if (n == q.length) resize(2 * q.length);
		q[last++] = item;//add first ,then increment
		if (last == q.length) last = 0;// wrap-around
		n++;
	}

	public Item dequeue()
	{
		if(isEmpty()) throw new NoSuchElementException("Queue underflow");//underflow堆栈下溢
		Item item = q[first];
		q[first] = null;// to avoid loitering 指针游离;without reference ,but wasted left
		n--;
		first++；
		if (first == q.length) first = 0;// wrap-around
		// shrink size of array if necessary
		if (n > 0 && n == q.length / 4 ) resize(q.length /2); // 25%-100%
		return item;

	}
	 /**
	* Returns the item least recently added to this queue.
	*
	* @return the item least recently added to this queue
	* @throws java.util.NoSuchElementException if this queue is empty
	*/
	public Item peek() {
		if (isEmpty()) throw new NoSuchElementException("Queue underflow");
		return q[first];
	}

	public static void main(String[] args){
		ResizingArrayQueue<String> queue = new ResizingArrayQueue<String>();
		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 the queue");
		}
	}

3 Generics | 泛型

Java 泛型（generics）是 JDK 5 中引入的一个新特性, 泛型提供了编译时类型安全检测机制，该机制允许程序员在编译时检测到非法的类型。泛型的本质是参数化类型，也就是说所操作的数据类型被指定为一个参数。

假定我们有这样一个需求：写一个排序方法，能够对整型数组、字符串数组甚至其他任何类型的数组进行排序，该如何实现？答案是可以使用 Java 泛型。

We implemented: StackOfStrings.

We also want: StackOfURLs, StackOfInts, StackOfVans, ….

Principles

• Welcome compile-time errors;
• Avoid run-time errors.

Generic stack:linked-list implementation

Generic stack:array implementation

the ugly cast //Cast - 强制类型转换，如果不匹配，容易发生runtime error

generic array creation not allowed in Java

What to do about primitive types?

• wrapped type
  
  Stack<Integer>stack=newStack<Integer>();
• autoboxing

4 Iterators | 迭代器

Purpose: elegant client code.

这种模式用于顺序访问集合对象的元素，不需要知道集合对象的底层表示。

allow the client to iterate through the items

without revealing the internal representation of the stack.（whether link-list or resizing array stack）

Stack iterator: linked-list implementation

Stack iterator: array implementation