Java栈实现

栈数组实现一：优点：入栈和出栈速度快，缺点：长度有限(有时候这也不能算是个缺点)

public class Stack {
	private int top = -1;
	private Object[] objs;

	public Stack(int capacity) throws Exception{
		if(capacity < 0)
			throw new Exception("Illegal capacity:"+capacity);
		objs = new Object[capacity];
	}

	public void push(Object obj) throws Exception{
		if(top == objs.length - 1)
			throw new Exception("Stack is full!");
		objs[++top] = obj;
	}

	public Object pop() throws Exception{
		if(top == -1)
			throw new Exception("Stack is empty!");
		return objs[top--];
	}

	public void dispaly(){
		System.out.print("bottom -> top: | ");
		for(int i = 0 ; i <= top ; i++){
			System.out.print(objs[i]+" | ");
		}
		System.out.print("\n");
	}

	public static void main(String[] args) throws Exception{
		Stack s = new Stack(2);
		s.push(1);
		s.push(2);
		s.dispaly();
		System.out.println(s.pop());
		s.dispaly();
		s.push(99);
		s.dispaly();
		s.push(99);
	}
}

bottom -> top: | 1 | 2 |
2
bottom -> top: | 1 |
bottom -> top: | 1 | 99 |
Exception in thread "main" java.lang.Exception: Stack is full!
	at Stack.push(Stack.java:17)
	at Stack.main(Stack.java:44)

数据项入栈和出栈的时间复杂度都为常数O(1)

栈数组实现二：优点：无长度限制，缺点：入栈慢

import java.util.Arrays;

public class UnboundedStack {
	private int top = -1;
	private Object[] objs;

	public UnboundedStack() throws Exception{
		this(10);
	}

	public UnboundedStack(int capacity) throws Exception{
		if(capacity < 0)
			throw new Exception("Illegal capacity:"+capacity);
		objs = new Object[capacity];
	}

	public void push(Object obj){
		if(top == objs.length - 1){
			this.enlarge();
		}
		objs[++top] = obj;
	}

	public Object pop() throws Exception{
		if(top == -1)
			throw new Exception("Stack is empty!");
		return objs[top--];
	}

	private void enlarge(){
		int num = objs.length/3;
		if(num == 0)
			num = 1;
		objs = Arrays.copyOf(objs, objs.length + num);
	}

	public void dispaly(){
		System.out.print("bottom -> top: | ");
		for(int i = 0 ; i <= top ; i++){
			System.out.print(objs[i]+" | ");
		}
		System.out.print("\n");
	}

	public static void main(String[] args) throws Exception{
		UnboundedStack us = new UnboundedStack(2);
		us.push(1);
		us.push(2);
		us.dispaly();
		System.out.println(us.pop());
		us.dispaly();
		us.push(99);
		us.dispaly();
		us.push(99);
		us.dispaly();
	}
}

bottom -> top: | 1 | 2 |
2
bottom -> top: | 1 |
bottom -> top: | 1 | 99 |
bottom -> top: | 1 | 99 | 99 |

由于该栈是由数组实现的，数组的长度是固定的，当栈空间不足时，必须将原数组数据复制到一个更长的数组中，考虑到入栈时或许需要进行数组复制，平均需要复制N/2个数据项，故入栈的时间复杂度为O(N)，出栈的时间复杂度依然为O(1)

栈单链表实现：没有长度限制，并且出栈和入栈速度都很快

public class LinkedList {
	private class Data{
		private Object obj;
		private Data next = null;

		Data(Object obj){
			this.obj = obj;
		}
	}

	private Data first = null;

	public void insertFirst(Object obj){
		Data data = new Data(obj);
		data.next = first;
		first = data;
	}

	public Object deleteFirst() throws Exception{
		if(first == null)
			throw new Exception("empty!");
		Data temp = first;
		first = first.next;
		return temp.obj;
	}

	public void display(){
		if(first == null)
			System.out.println("empty");
		System.out.print("top -> bottom : | ");
		Data cur = first;
		while(cur != null){
			System.out.print(cur.obj.toString() + " | ");
			cur = cur.next;
		}
		System.out.print("\n");
	}
}

public class LinkedListStack {
	private LinkedList ll = new LinkedList();

	public void push(Object obj){
		ll.insertFirst(obj);
	}

	public Object pop() throws Exception{
		return ll.deleteFirst();
	}

	public void display(){
		ll.display();
	}

	public static void main(String[] args) throws Exception{
		LinkedListStack lls = new LinkedListStack();
		lls.push(1);
		lls.push(2);
		lls.push(3);
		lls.display();
		System.out.println(lls.pop());
		lls.display();
	}
}

top -> bottom : | 3 | 2 | 1 |
3
top -> bottom : | 2 | 1 | 

数据项入栈和出栈的时间复杂度都为常数O(1)