Coursera普林斯顿算法课第二次作业

Deque: 使用LinkedList和哨兵节点即可。

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

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

public class Deque<Item> implements Iterable<Item> {
	
	private int num; // number of nodes
	private Node first;
	private Node last;
		
	public Deque(){
		this.num = 0;
		this.first = null;
		this.last = null;	
	}
	
	private class Node{
		private Item item;
		private Node next;
		private Node prev;
	}
	
	public boolean isEmpty(){
		return first == null;
	}
	
	public int size(){
		return num;
	}
	
	// add the item to the front
	public void addFirst(Item item){
		if(item == null) throw new IllegalArgumentException();
		
		Node oldfirst = first;
		first = new Node();
		first.item = item;
		first.prev = null;
		
		if(oldfirst == null){
			last = first;
			first.next = null;	
		}else{
			first.next = oldfirst;
			oldfirst.prev = first;
			
		}
		num++;
	}
	
	// add the item to the end
	public void addLast(Item item){
		if(item == null) throw new IllegalArgumentException();
		
		Node oldlast = last;
		last = new Node();
		last.item = item;
		last.next = null;
		
		if(isEmpty()){
			first = last;
			last.prev = null;
		}else{
			oldlast.next = last;
			last.prev = oldlast;
		}
		num++;	
	}
	
	// remove and return the item from the front
	public Item removeFirst(){
		if(isEmpty()) throw new NoSuchElementException();
		
		Item it = first.item;
		first = first.next;
		num--;
		
		if(num == 0) last = null;
		else first.prev = null;
		
		return it;
		
	}
	
	public Item removeLast(){
		if(isEmpty()) throw new NoSuchElementException();
		
		Item it = last.item;
		last = last.prev;
		num--;

		if(num == 0) first = null;
		else last.next = null;
		
		return it;
	}
	
	// return an iterator over items in order from front to end
	public Iterator<Item> iterator(){	
		return new DequeIterator();	
	}
	
	private class DequeIterator implements Iterator<Item>{
		
		private Node current = first;

		public boolean hasNext() {return current != null;}
		public void remove() {throw new UnsupportedOperationException();}
			
		public Item next() {
			
			if(!hasNext()) throw new NoSuchElementException();
			
			Item it = current.item;
			current = current.next;
			return it;
		}
	}
	
	// test
	public static void main(String[] args){
		Deque<String> dq = new Deque<String>();
		dq.addFirst("I");
		dq.addFirst("am");
		dq.addFirst("Hugh");
		dq.addLast("I");
		dq.addLast("Love");
		dq.addLast("Gals");
		
		Stopwatch sw = new Stopwatch();
		
		for(String s:dq){ StdOut.println(s);}
		StdOut.println("remove first: "+dq.removeFirst());
		StdOut.println("remove last: "+dq.removeLast());
		for(String s:dq){ StdOut.println(s);}
		
		StdOut.println("elapsed CPU time: "+sw.elapsedTime());
	}

}

RandomizedQueue: 使用ResizedArray实现，关键在于在每次dequeue操作后需要把末端节点存储的item转存到被随机移除item的节点处，然后将尾端节点设为null，避免loitering。同时也能避免wrap-around的问题，即不会出现数组前几项被dequeue后值为null的情况。

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

import edu.princeton.cs.algs4.StdRandom;

public class RandomizedQueue<Item> implements Iterable<Item> {
	
	private Item[] que;
	private int num; // number of elements on queue
	
	public RandomizedQueue(){
		que = (Item[]) new Object[2];
		num = 0;
	}
	
	public boolean isEmpty(){
		return num == 0;
	}
	
	public int size(){
		return num;
	}
	
	private void resize(int capacity){
		assert capacity >= num;
		Item[] temp = (Item[]) new Object[capacity];
		
		for(int i=0; i<num; i++){
			temp[i] = que[i];
		}
		
		que = temp;
	}
	
	public void enqueue(Item item){
		if(item == null) throw new IllegalArgumentException();
		if(num == que.length) resize(2 * que.length);
		
		que[num++] = item;
	}
	
	// remove and return a random item
	public Item dequeue(){
		if(isEmpty()) throw new NoSuchElementException();
		
		int ch = StdRandom.uniform(num);
		Item it = que[ch];
		
		if(ch != num-1) que[ch] = que[num-1];
		que[num-1] = null; // to avoid loitering
		num--;
		
		if(num > 0 && num == que.length / 4) resize(que.length / 2);
		return it;
	}
	
	// return a random item but not remove it
	public Item sample(){
		if(isEmpty()) throw new NoSuchElementException();
		
		int ch = StdRandom.uniform(num);
		return que[ch];
	}

	// return an independent iterator over items in random order
	public Iterator<Item> iterator() {
		return new RandomizedQueueIterator();
	}
	
	private class RandomizedQueueIterator implements Iterator<Item>{
		
		private Item[] rdq; // independent iterator
		private int current;
		
		public RandomizedQueueIterator(){
			
			rdq = (Item[]) new Object[num];
			current = 0;
			
			for(int k=0; k<num; k++){
				rdq[k] = que[k];
			}
			
			StdRandom.shuffle(rdq);
		}
		
		public boolean hasNext() {return current < num;}
		public void remove() {throw new UnsupportedOperationException();}
			
		public Item next() {
			
			if(!hasNext()) throw new NoSuchElementException();
			
			Item it = rdq[current];
			current++;
			return it;
		}
	}
	
}

Permutation：注意k是从命令行获取的，以及不要有多余的输出。在参考资料中提到了新建字符串数组和使用readStrings函数获得bonus的方法，不过实际测试时却发现如下错误：

[ERROR] Permutation.java:11: Do not declare arrays in this program. [Performance]

[ERROR] Permutation.java:11:44: Do not call 'StdIn.readAllStrings()' in this program. Use only 'StdIn.readString()'. 

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

public class Permutation {
	
	public static void main(String[] args){
		
		RandomizedQueue<String> r = new RandomizedQueue<String>();
		//StdOut.println(" size: " + r.size());
		
		// input from command line
		int k = Integer.parseInt(args[0]);
		
		String s;
		//StdOut.println("Input strings: "); -> no redundant output!
		
		// use ctrl+z to end input
		while(!StdIn.isEmpty()){
			//r.enqueue(s); -> the last string won't be enqueued!
			s = StdIn.readString();
			r.enqueue(s);
		}
		
		int count = 0;
		for(String ss:r){
			if(count < k){
				StdOut.println(ss);
				count++;
			}else break;
		}
		
	}

}

references:

https://blog.csdn.net/sesiria/article/details/52586467

https://segmentfault.com/a/1190000005345079

https://blog.csdn.net/tumaolin94/article/details/43448485