栈

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1.抽象数据类型

/**
 * 
 * 栈接口
 *
 */
public interface Stack {
    //返回堆栈大小
    public int getSize();
    //判断是否为空
    public boolean isEmpty();
    //e入栈
    public void push(Object e);
    //栈顶元素出栈
    public Object pop() throws StackEmptyException;
    //取栈顶元素
    public Object peek() throws StackEmptyException;
}

class StackEmptyException extends RuntimeException{
    public StackEmptyException(String err) {
        super(err);
    }
}

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2.栈的顺序存储实现

/**
 * 
 * Stack的顺序存储实现
 *
 */
public class StackArray implements Stack {

    private final int LEN = 8;//默认
    private Object[] elements;
    private int top;//栈顶指针

    public StackArray() {
        top = -1;
        elements = new Object[LEN];
    }

    @Override
    public int getSize() {
        return top+1;
    }

    @Override
    public boolean isEmpty() {
        return top<0;
    }

    @Override
    public void push(Object e) {
        if(getSize() >= elements.length){
            expandSpace();
        }
        elements[++top] = e;
    }
    private void expandSpace(){
        Object[] a = new Object[elements.length*2];
        for(int i=0;i<elements.length;i++){
            a[i] = elements[i];
        }
        elements = a;
    }

    @Override
    public Object pop() throws StackEmptyException {
        if(getSize() < 1){
            throw new StackEmptyException("错误，堆栈为空");
        }
        Object obj = elements[top];
        elements[top--] = null;
        return obj;
    }

    @Override
    public Object peek() throws StackEmptyException {
        if(getSize() < 1){
            throw new StackEmptyException("错误，堆栈为空");
        }
        return elements[top];
    }

}

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3.栈的链式存储实现

/**
 * 
 * 栈的链式存储
 *
 */
public class StackSLinked implements Stack {

    private SLNode top;//首结点
    private int size;

    public StackSLinked() {
        top = null;
        size = 0;
    }

    @Override
    public int getSize() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public void push(Object e) {
        SLNode q = new SLNode(e,top);
        top = q;
        size++;
    }

    @Override
    public Object pop() throws StackEmptyException {
        if(size < 1){
            throw new StackEmptyException("错误，堆栈为空");
        }
        Object obj = top.getData();
        top = top.getNext();
        size--;
        return obj;
    }

    @Override
    public Object peek() throws StackEmptyException {
        if(size < 1){
            throw new StackEmptyException("错误，堆栈为空");
        }
        return top.getData();
    }

}

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4.应用——进制转换