Stack and Queue 栈和队列

栈：

接口定义：

public interface Stack<T> extends Iterable<T> {/** * Check if the stack is empty. * @return True if it is empty. False otherwise. */public boolean isEmpty();/** * Get the size of the stack. * @return Size */public int size();/** * Get the top of the stack. * @return Top of the stack. */public T peek();/** * Push an item to the stack  * @param item Item to be pushed. */public void push(T item);/** * Pop an item from the stack. * @param item Item to be popped. * @return Item */public T pop();/** *  * Search the position of the item in the stack * @param item Item * @return Position of item. -1 if non-exists. */public int search(T item);}

链表实现：

public class LinkedListStack<T> implements Stack<T> {private Node top;private int size;@Overridepublic boolean isEmpty() {return size == 0;}@Overridepublic int size() {return size;}@Overridepublic T peek() {if (size == 0) {return null;}return top.item;}@Overridepublic void push(T item) {Node node = new Node(item);node.next = top;top = node;size++;}@Overridepublic T pop() {if (size == 0) {return null;}T item = top.item;top = top.next;size--;return item;}@Overridepublic int search(T item) {if (size == 0 || item == null) {return -1;}Node current = top;int index = 0;while (current != null) {if (current.item.equals(item)) {return index;}current = current.next;index++;}return -1;}@Overridepublic Iterator<T> iterator() {return new Iterator<T>() {Node current = top;@Overridepublic boolean hasNext() {return current != null;}@Overridepublic T next() {T item = current.item;current = current.next;return item;}};}private class Node {T item;Node next;Node(T item) {this.item = item;}}}

数组实现：

public class ArrayStack<T> implements Stack<T> {private T[] stack;private int top;@SuppressWarnings("unchecked")public ArrayStack() {stack = (T[]) new Object[4];top = -1;}@Overridepublic boolean isEmpty() {return top == -1;}@Overridepublic int size() {return top + 1;}@Overridepublic T peek() {if (top == -1) {return null;}return stack[top];}@Overridepublic void push(T item) {if (top + 1 == stack.length) {resize(stack.length * 2);}stack[++top] = item;}@Overridepublic T pop() {if (top == -1) {return null;}T item = stack[top];stack[top--] = null;if (stack.length > 4 && top + 1 == stack.length / 4) {resize(stack.length / 2);}return item;}@Overridepublic int search(T item) {if (item == null) {return -1;}int current = top;while (current >= 0) {if (stack[current].equals(item)) {return top - current;}current--;}return -1;}@SuppressWarnings("unchecked")private void resize(int size) {T[] newStack = (T[]) new Object[size];System.arraycopy(stack, 0, newStack, 0, top + 1);stack = newStack;}@Overridepublic Iterator<T> iterator() {return new Iterator<T>() {private int current = top;@Overridepublic boolean hasNext() {return current != -1;}@Overridepublic T next() {return stack[current--];}};}}

队列：

接口定义：

public interface Queue<T> extends Iterable<T> {/** * Check if the queue is empty. *  * @return True if the queue is empty. False otherwise. */public boolean isEmpty();/** * Get the size of the queue. *  * @return Size */public int size();/** * Get the head of the queue. *  * @return Head */public T peek();/** * Add an item to the queue. *  * @param item *            Item */public void add(T item);/** * Poll the head of the queue. *  * @return Head */public T poll();/** *  * Search the position of the item in the queue * @param item Item * @return Position of item. -1 if non-exists. */public int search(T item);}

链表实现：

public class LinkedListQueue<T> implements Queue<T> {private Node dummyHead;private Node tail;private int size;public LinkedListQueue() {dummyHead = new Node(null);tail = dummyHead;}@Overridepublic boolean isEmpty() {return size == 0;}@Overridepublic int size() {return size;}@Overridepublic T peek() {if (size == 0) {return null;}return dummyHead.next.item;}@Overridepublic void add(T item) {Node node = new Node(item);tail.next = node;tail = tail.next;size++;}@Overridepublic T poll() {if (size == 0) {return null;}T item = dummyHead.next.item;dummyHead.next = dummyHead.next.next;size--;if(size == 0) {tail = dummyHead;}return item;}@Overridepublic int search(T item) {if (size == 0 || item == null) {return -1;}Node current = dummyHead.next;int index = 0;while (current != null) {if (current.item.equals(item)) {return index;}current = current.next;index++;}return -1;}@Overridepublic Iterator<T> iterator() {return new Iterator<T>() {Node current = dummyHead.next;@Overridepublic boolean hasNext() {return current != null;}@Overridepublic T next() {T item = current.item;current = current.next;return item;}};}private class Node {T item;Node next;Node(T item) {this.item = item;}}}

数组实现：

public class ArrayQueue<T> implements Queue<T> {private T[] queue;private int head;private int tail;private int size;@SuppressWarnings("unchecked")public ArrayQueue() {queue = (T[]) new Object[4];tail = -1;}@Overridepublic boolean isEmpty() {return size == 0;}@Overridepublic int size() {return size;}@Overridepublic T peek() {if (size == 0) {return null;}return queue[head];}@Overridepublic void add(T item) {if (tail + 1 == queue.length) {if (!refresh()) {resize(queue.length * 2);}}queue[++tail] = item;size++;}@Overridepublic T poll() {if (size == 0) {return null;}T item = queue[head];queue[head++] = null;size--;if (size == 0) {head = 0;tail = -1;}if (queue.length > 4 && size == queue.length / 4) {resize(queue.length / 2);}return item;}@Overridepublic int search(T item) {if (size == 0 || item == null) {return -1;}int current = head;while (current <= tail) {if (queue[current].equals(item)) {return current - head;}current++;}return -1;}@Overridepublic Iterator<T> iterator() {return new Iterator<T>() {int current = head;@Overridepublic boolean hasNext() {return current <= tail;}@Overridepublic T next() {return queue[current++];}};}@SuppressWarnings("unchecked")private void resize(int capacity) {T[] newQueue = (T[]) new Object[capacity];System.arraycopy(queue, head, newQueue, 0, size);queue = newQueue;head = 0;tail = size - 1;}private boolean refresh() {if (head == 0) {return false;}if (size >= 3 * queue.length / 4) {return false;}for (int i = head, j = 0; i <= tail; i++, j++) {queue[j] = queue[i];}head = 0;tail = size - 1;return true;}}

总结：
数组实现要注意容量的变化。

实现自定义集合的Iterate：

1. Implement Iterable 接口，返回一个iterator

2. 内建类implement Iterator，实现iterator。

注意，foreach 实际是调用了iterator。

Java 不支持new generic 数组，使用cast实现。T[] array = (T[]) new Object[3];
ArrayList用的就是resizing array。LinkedList 是用的linked list。

Stack栈的特性是先入后出，Queue队列的特性是先入先出。Deque的特性是双向进出。addFirst加在头，addLast加在尾。removeFirst从头出，removeLast从尾出。

Deque作为Stack是addFirst，removeFirst。头进头出。Deque<T> stack = new ArrayDeque<T>();

Deque作为Queue是addLast，removeFirst。尾进头出。Queue<T> queue = new ArrayDeque<T>();

应用：

1. Dijkstra's two stack algorithm 迪杰斯特拉双栈法计算表达式

public static double calculate(char[] input) {Stack<Character> operation = new Stack<Character>();Stack<Double> value = new Stack<Double>();for (char i : input) {if (i == '(') {;} else if (i == '+') {operation.push(i);} else if (i == '*') {operation.push(i);} else if (i == ')') {char op = operation.pop();if (op == '+') {value.push(value.pop() + value.pop());} else if (op == '*') {value.push(value.pop() * value.pop());}} else {value.push((double) (i - '0'));}}return value.pop();}

两个栈，一个存操作，一个存值。如果遇到）时，值栈弹出两个值，操作栈弹出一个操作，进行计算，结果存入值栈中。

例如 ：（4+（1+2））

2

1  +

4  +

1. 遇到第一个），弹出2, 1, + 进行计算（2+1），结果存入值栈

3

4   +

2. 遇到第二个），弹出3, 4, + 进行聚散（3 + 4），结果存入值栈

7

3. 得到结果 7

2. 十进制转二进制

public static String toBinary(int num) {if (num == 0) {return "0";}Stack<Integer> result = new Stack<Integer>();while (num > 0) {result.push(num % 2);num /= 2;}StringBuilder sb = new StringBuilder();while (!result.isEmpty()) {sb.append(result.pop());}return sb.toString();}

Java Stack API 的iterator（for each）是按入栈顺序next，而不是出栈顺序。