算法 - 栈和队列

• 算法 - 栈和队列
  • 栈
    • 1. 数组实现
    • 2. 链表实现
  • 队列

栈

public interface MyStack<Item> extends Iterable<Item> {MyStack<Item> push(Item item);Item pop() throws Exception;boolean isEmpty();int size();}

1. 数组实现

public class ArrayStack<Item> implements MyStack<Item> {// 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;@Overridepublic MyStack<Item> push(Item item) {check();a[N++] = item;return this;}@Overridepublic 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;}@Overridepublic boolean isEmpty() {return N == 0;}@Overridepublic int size() {return N;}@Overridepublic Iterator<Item> iterator() {// 返回逆序遍历的迭代器return new Iterator<Item>() {private int i = N;@Overridepublic boolean hasNext() {return i > 0;}@Overridepublic 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。

public class ListStack<Item> implements MyStack<Item> {private Node top = null;private int N = 0;private class Node {Item item;Node next;}@Overridepublic MyStack<Item> push(Item item) {Node newTop = new Node();newTop.item = item;newTop.next = top;top = newTop;N++;return this;}@Overridepublic Item pop() throws Exception {if (isEmpty()) {throw new Exception("stack is empty");}Item item = top.item;top = top.next;N--;return item;}@Overridepublic boolean isEmpty() {return N == 0;}@Overridepublic int size() {return N;}@Overridepublic Iterator<Item> iterator() {return new Iterator<Item>() {private Node cur = top;@Overridepublic boolean hasNext() {return cur != null;}@Overridepublic 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。

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;}@Overridepublic boolean isEmpty() {return N == 0;}@Overridepublic int size() {return N;}@Overridepublic 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;}@Overridepublic 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;}@Overridepublic Iterator<Item> iterator() {return new Iterator<Item>() {Node cur = first;@Overridepublic boolean hasNext() {return cur != null;}@Overridepublic Item next() {Item item = cur.item;cur = cur.next;return item;}};}
}