背包

定义

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• 背包是一种不支持从中删除元素的集合数据类型
• 它的目的就是帮助用例收集元素并迭代遍历所有收集到的元素
• 用例也可以检查背包是否为空或者获取背包中元素的数量
• 迭代的顺序不确定且与用例无关

Java代码实现

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• 源码

import java.util.Iterator;

public class Bag<Item> implements Iterable<Item> {
    private Node first;// 链表的首结点
    private int N;// 元素数量

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

    public int size() {
        return N;
    }

    // 和Stack的push()方法完全相同
    public void add(Item item) {
        Node oldFirst = first;
        first = new Node();
        first.item = item;
        first.next = oldFirst;
        N++;
    }

    public Iterator<Item> iterator() {
        return new ListIterator();
    }

    private class Node {
        Item item;
        Node next;
    }

    private class ListIterator implements Iterator<Item> {
        private Node current = first;

        public boolean hasNext() {
            return null != current;
        }

        public Item next() {
            Item item = current.item;
            current = current.next;
            return item;
        }
    }
}

• 测试用例

public class Test {
    public static void main(String[] args) {

        Bag<Double> numbers = new Bag<Double>();
        double[] nums = {100, 99, 101, 120, 98, 107, 109, 81, 101, 90};
        for (double num : nums) numbers.add(num);
        int N = numbers.size();
        double sum = 0.0;
        for (double x : numbers)
            sum += x;
        double mean = sum / N;

        sum = 0.0;
        for (double x : numbers)
            sum += (x - mean) * (x - mean);
        double std = Math.sqrt(sum / (N - 1));
        System.out.printf("Mean: %.2f\n", mean);
        System.out.printf("Std dev: %.2f\n", std);
    }
}

• 测试结果

Mean: 100.60
Std dev: 10.51