算法 - 棧和隊列

• 棧
  • 1. 數組實現
  • 2. 鏈表實現
• 隊列

棧

public interface MyStack<Item> extends Iterable<Item> {

    MyStack<Item> push(Item item);

    Item pop() throws Exception;

    boolean isEmpty();

    int size();

}

1. 數組實現

public class ArrayStack<Item> implements MyStack<Item> {

    // 棧元素數組，只能通過轉型來創建泛型數組
    private Item[] a = (Item[]) new Object[1];

    // 元素數量
    private int N = 0;


    @Override
    public MyStack<Item> push(Item item) {
        check();
        a[N++] = item;
        return this;
    }


    @Override
    public Item pop() throws Exception {

        if (isEmpty()) {
            throw new Exception("stack is empty");
        }

        Item item = a[--N];

        check();

        // 避免對象遊離
        a[N] = null;

        return item;
    }


    private void check() {

        if (N >= a.length) {
            resize(2 * a.length);

        } else if (N > 0 && N <= a.length / 4) {
            resize(a.length / 2);
        }
    }


    /**
     * 調整數組大小，使得棧具有伸縮性
     */
    private void resize(int size) {

        Item[] tmp = (Item[]) new Object[size];

        for (int i = 0; i < N; i++) {
            tmp[i] = a[i];
        }

        a = tmp;
    }


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


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


    @Override
    public Iterator<Item> iterator() {

        // 返回逆序遍歷的迭代器
        return new Iterator<Item>() {

            private int i = N;

            @Override
            public boolean hasNext() {
                return i > 0;
            }

            @Override
            public Item next() {
                return a[--i];
            }
        };

    }
}

2. 鏈表實現

需要使用鏈表的頭插法來實現，因爲頭插法中最後壓入棧的元素在鏈表的開頭，它的 next 指針指向前一個壓入棧的元素，在彈出元素時就可以通過 next 指針遍歷到前一個壓入棧的元素從而讓這個元素成爲新的棧頂元素。

public class ListStack<Item> implements MyStack<Item> {

    private Node top = null;
    private int N = 0;


    private class Node {
        Item item;
        Node next;
    }


    @Override
    public MyStack<Item> push(Item item) {

        Node newTop = new Node();

        newTop.item = item;
        newTop.next = top;

        top = newTop;

        N++;

        return this;
    }


    @Override
    public Item pop() throws Exception {

        if (isEmpty()) {
            throw new Exception("stack is empty");
        }

        Item item = top.item;

        top = top.next;
        N--;

        return item;
    }


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


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


    @Override
    public Iterator<Item> iterator() {

        return new Iterator<Item>() {

            private Node cur = top;


            @Override
            public boolean hasNext() {
                return cur != null;
            }


            @Override
            public Item next() {
                Item item = cur.item;
                cur = cur.next;
                return item;
            }
        };

    }
}

隊列

下面是隊列的鏈表實現，需要維護 first 和 last 節點指針，分別指向隊首和隊尾。

這裏需要考慮 first 和 last 指針哪個作爲鏈表的開頭。因爲出隊列操作需要讓隊首元素的下一個元素成爲隊首，所以需要容易獲取下一個元素，而鏈表的頭部節點的 next 指針指向下一個元素，因此可以讓 first 指針鏈表的開頭。

public interface MyQueue<Item> extends Iterable<Item> {

    int size();

    boolean isEmpty();

    MyQueue<Item> add(Item item);

    Item remove() throws Exception;
}

public class ListQueue<Item> implements MyQueue<Item> {

    private Node first;
    private Node last;
    int N = 0;


    private class Node {
        Item item;
        Node next;
    }


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


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


    @Override
    public MyQueue<Item> add(Item item) {

        Node newNode = new Node();
        newNode.item = item;
        newNode.next = null;

        if (isEmpty()) {
            last = newNode;
            first = newNode;
        } else {
            last.next = newNode;
            last = newNode;
        }

        N++;
        return this;
    }


    @Override
    public Item remove() throws Exception {

        if (isEmpty()) {
            throw new Exception("queue is empty");
        }

        Node node = first;
        first = first.next;
        N--;

        if (isEmpty()) {
            last = null;
        }

        return node.item;
    }


    @Override
    public Iterator<Item> iterator() {

        return new Iterator<Item>() {

            Node cur = first;


            @Override
            public boolean hasNext() {
                return cur != null;
            }


            @Override
            public Item next() {
                Item item = cur.item;
                cur = cur.next;
                return item;
            }
        };
    }
}