* [栈](#栈) * [1. 数组实现](#1-数组实现) * [2. 链表实现](#2-链表实现) * [队列](#队列) # 栈 ```java public interface MyStack extends Iterable { MyStack push(Item item); Item pop() throws Exception; boolean isEmpty(); int size(); } ``` ## 1. 数组实现 ```java public class ArrayStack implements MyStack { // 栈元素数组，只能通过转型来创建泛型数组 private Item[] a = (Item[]) new Object[1]; // 元素数量 private int N = 0; @Override public MyStack 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(); // 避免对象游离 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); } } /** * 调整数组大小，使得栈具有伸缩性 */ 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 iterator() { // 返回逆序遍历的迭代器 return new Iterator() { private int i = N; @Override public boolean hasNext() { return i > 0; } @Override public Item next() { return a[--i]; } }; } } ``` ## 2. 链表实现需要使用链表的头插法来实现，因为头插法中最后压入栈的元素在链表的开头，它的 next 指针指向前一个压入栈的元素，在弹出元素时就可以通过 next 指针遍历到前一个压入栈的元素从而让这个元素成为新的栈顶元素。 ```java public class ListStack implements MyStack { private Node top = null; private int N = 0; private class Node { Item item; Node next; } @Override public MyStack 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 iterator() { return new Iterator() { private Node cur = top; @Override public boolean hasNext() { return cur != null; } @Override public Item next() { Item item = cur.item; cur = cur.next; return item; } }; } } ``` # 队列下面是队列的链表实现，需要维护 first 和 last 节点指针，分别指向队首和队尾。这里需要考虑 first 和 last 指针哪个作为链表的开头。因为出队列操作需要让队首元素的下一个元素成为队首，所以需要容易获取下一个元素，而链表的头部节点的 next 指针指向下一个元素，因此可以让 first 指针链表的开头。 ```java public interface MyQueue extends Iterable { int size(); boolean isEmpty(); MyQueue add(Item item); Item remove() throws Exception; } ``` ```java public class ListQueue implements MyQueue { 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 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 iterator() { return new Iterator() { Node cur = first; @Override public boolean hasNext() { return cur != null; } @Override public Item next() { Item item = cur.item; cur = cur.next; return item; } }; } } ```

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