算法系列（一）背包、队列和栈（Java实现）

前言：算法第四版1.3节 背包、队列和栈 学习总结

Stack：

import java.util.Iterator;/** * 动态调整数组大小的栈 * 功能： LIFO * @author Chuanjie * @param <T> * */public class ResizingArrayStack<T> implements Iterable<T> {private T[] a = (T[])new Object[1];;private int n = 0;private void resize(int sz){T[] temp = (T[])new Object[sz];for(int i = 0;i < n;i++)temp[i] = a[i];a = temp;}public boolean isEmpty(){return n == 0;}public int size(){return n;}public void push(T item){if(n == a.length)resize(2*a.length);a[n++] = item;}public T pop(){T item = a[--n];a[n] = null;//释放引用， 避免对象游离，垃圾回收器可以将其回收if(n > 0 && n == a.length/4)resize(a.length/2);return item;}@Overridepublic Iterator<T> iterator() {return new ReverseArrayIterator();}private class ReverseArrayIterator implements Iterator<T>{private int i = n;@Overridepublic boolean hasNext(){return i > 0;}@Overridepublic T next() {return a[--i];}@Overridepublic void remove() {}}}

import java.util.Iterator;/** * 链表实现栈 * 功能：LIFO * @author Chuanjie * */public class Stack <T> implements Iterable<T>{private Node first;private int n;private class Node{T item;Node next;}public boolean isEmpty(){return first == null;} public int size(){ return n; }  public void push(T item){ Node oldfirst = first; first = new Node(); first.item = item; first.next = oldfirst; n++; }  public T pop(){ T item = first.item; first = first.next; n--; return item; }@Overridepublic Iterator<T> iterator() {return new ListIterator();}private class  ListIterator implements Iterator<T>{private Node current = first;@Overridepublic boolean hasNext() {return current != null;}@Overridepublic T next() {T item = current.item;current = current.next;return item;}@Overridepublic void remove() {}} }

Queue:

import java.util.Iterator;/** * 链表实现队列 * 功能：FIFO * @author Chuanjie * */public class Queue<T> implements Iterable<T> {private Node first;private Node last;private int n;private class Node{T item;Node next;}public boolean isEmpty(){return first == null;}public int size(){return n;}public void enqueue(T item){ //向表尾添加元素Node oldlast = last;last = new Node();last.item = item;last.next = null;if(isEmpty())first = last; //找到lastelse oldlast.next = last;n++;}public T dequeue(){  //从表头删除元素T item = first.item;first = first.next;if(isEmpty()) last = null;n--;return item;}@Overridepublic Iterator<T> iterator() {return null;}}

Bag:

import java.util.Iterator;/** * 链表实现背包 * 功能：保存添加的元素 * @author Chuanjie * * @param <T> */public class Bag <T> implements Iterable<T>{private Node first;private int n;private class Node{T item;Node next;}public boolean isEmpty(){return first == null;} public int size(){ return n; }  public void add(T item){ Node oldfirst = first; first = new Node(); first.item = item; first.next = oldfirst; n++; } @Overridepublic Iterator<T> iterator() {return new ListIterator();}private class  ListIterator implements Iterator<T>{private Node current = first;@Overridepublic boolean hasNext() {return current != null;}@Overridepublic T next() {T item = current.item;current = current.next;return item;}@Overridepublic void remove() {}} }

总结：1. 结构化存储中，链表（链式存储）是数组（顺序存储）的重要替代方式：可以处理任意类型的数据、所需空间总是和集合大小成正比、一次操作的时间和集合大小无关。

2. 学习底层实现，尝试写API是深入理解的必经之路，Don't use a library until you understand its API!