数据结构《栈和队列》

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提示：关于栈和队列的实现其实很简单，基本上是对之前的顺序表和链表的一种应用，代码部分也不难。主要的是对栈和队列的实际运用。

一、什么是栈？

可以理解为一个竖过来的数组、顺序表，同时这个数组只能将末尾的元素先推出来，再推出来前面的元素，也就是我们所说的，先进后出、后进先出。

栈的实例化

Stack<E> stack = new Stack<>();

1.1 栈的模拟实现

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自定义的接口，规范自定义的类,便于模拟实现

public interface IStack {//放入元素int push(int a);//推出栈顶元素int pop();//查看栈顶元素int peek();//栈是否为空boolean isEmpty(int[] array);//栈中的元素个数int size();//栈中寻找某个元素距离栈顶的距离int search(int a);
}

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push(int a) — 将元素放入栈中

public int push(int a) {isFull(array);array[useSide++] = a;return a;}

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peek() — 查看栈顶元素

public int peek() {try {if(isEmpty(array)){throw new EmptyException("空数组读取异常");}}catch (EmptyException e){e.printStackTrace();}return array[useSide - 1];}

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pop() — 出栈顶元素

public int pop() {int top = peek();useSide--;return top;}

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isEmpty(int[] array) — 判断栈是否为空

public boolean isEmpty(int[] array) {return array.length == useSide;}

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size() — 得到栈中的元素个数

public int size() {return useSide;}

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search(int a) — 得到栈中某个元素距离栈顶的距离，栈顶元素位置按1来计算

public int search(int a) {int cur = useSide - 1;int len = 1;while (0 <= cur){if(a == array[cur]){return len;}else {cur--;len++;}}return -1;}

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代码整合

public class MyStack implements IStack{private int[] array;public MyStack() {this.array = new int[10];}private int useSide;private void isFull(int[] check){if(isEmpty(check)){array = Arrays.copyOf(check,2*check.length);}}@Overridepublic int push(int a) {isFull(array);array[useSide++] = a;return a;}@Overridepublic int pop() {int top = peek();useSide--;return top;}@Overridepublic int peek() {try {if(isEmpty(array)){throw new EmptyException("空数组读取异常");}}catch (EmptyException e){e.printStackTrace();}return array[useSide - 1];}@Overridepublic boolean isEmpty(int[] array) {return array.length == useSide;}@Overridepublic int size() {return useSide;}@Overridepublic int search(int a) {int cur = useSide - 1;int len = 1;while (0 <= cur){if(a == array[cur]){return len;}else {cur--;len++;}}return -1;}
}

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1.2 关于栈的例题

例题1 有效括号

class Solution {public boolean isValid(String s) {Stack<Character> stack = new Stack<>();if(s == null){return false;}for(int i = 0; i < s.length(); i++){char cur = s.charAt(i);if(cur == '(' || cur == '{' || cur == '['){stack.push(cur);}else if(cur == ')' || cur == '}' || cur == ']'){if(stack.isEmpty()){return false;}if(cur == ')'&&stack.peek() == '(' || cur == '}'&&stack.peek() == '{' || cur == ']'&&stack.peek() == '['){stack.pop();}else{return false;}}else{return false;}}if(stack.isEmpty()){return true;}return false;}
}

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例题2 逆波兰表达式

public int evalRPN(String[] tokens) {Stack<String> stack = new Stack<>();for(int i = 0; i < tokens.length; i++){if(tokens[i].equals("+") || tokens[i].equals("-") || tokens[i].equals("*") || tokens[i].equals("/")){int m = Integer.parseInt(stack.pop());int n = Integer.parseInt(stack.pop());if(tokens[i].equals("+")){stack.push(String.valueOf(m+n));}else if(tokens[i].equals("-")){stack.push(String.valueOf(n-m));}else if(tokens[i].equals("*")){stack.push(String.valueOf(m*n));}else{stack.push(String.valueOf(n/m));}}else{stack.push(tokens[i]);}}return Integer.parseInt(stack.pop());}

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例题3 最小栈

public Stack<Integer> stack;public Stack<Integer> minStack;public MinStack() {stack = new Stack<>();minStack = new Stack<>();}public void push(int val) {stack.push(val);if(minStack.empty()){minStack.push(val);}else{int m = minStack.peek();if(m >= val){minStack.push(val);}}}public void pop() {if(stack.empty()){return;}int m = stack.pop();if(m == minStack.peek()){minStack.pop();}}public int top() {if(stack.empty()){return -1;}return stack.peek();}public int getMin() {if(minStack.empty()){return -1;}return minStack.peek();}

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例题4 栈的压入、弹出序列

public boolean IsPopOrder (int[] pushV, int[] popV) {// write code hereStack<Integer> stack = new Stack<>();int j = 0;for(int i = 0; i < pushV.length;i++){stack.push(pushV[i]);while(!stack.isEmpty() && stack.peek() == popV[j] && j < popV.length){stack.pop();j++;}}return stack.isEmpty();}

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二、什么是队列？

可以理解为一个双向链表(一个一个节点在排队)，同时这个链表只能将队头的元素先推出来，再推出来后面的元素，也就是我们所说的，先进先出、后进后出。

队列的实例化

Deque<E> stack = new ArrayDeque<>();//双端队列的线性实现
Deque<E> queue = new LinkedList<>();//双端队列的链式实现

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2.2 队列的模拟实现

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自定义的接口，规范自定义的类,便于模拟实现

public interface IQueue {//int offer(int a);int poll();int peek();boolean isEmpty();
}

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isEmpty() — 判断是否为空队列

@Overridepublic boolean isEmpty() {return useSide == 0;}

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offer(int a) — 从队尾放入元素

public int offer(int a) {ListNode listNode = new ListNode(a);listNode.val = a;if(isEmpty()){head = listNode;last = listNode;}else {last.next = listNode;listNode.pre = last;last = listNode;}useSide++;return a;}

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peek() — 查看队头元素

public int peek() {try {if(head == null){throw new EmptyException("空指针异常访问");}}catch (EmptyException e){e.printStackTrace();}return head.val;}

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poll() — 推出队头的元素

public int poll() {int fisrt = peek();head = head.next;useSide --;return fisrt;}

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代码整合

public class MyQueue implements IQueue{static class ListNode{int val;ListNode pre;ListNode next;public ListNode(int val) {this.val = val;}}private ListNode head;private ListNode last;private int useSide;@Overridepublic int offer(int a) {ListNode listNode = new ListNode(a);listNode.val = a;if(isEmpty()){head = listNode;last = listNode;}else {last.next = listNode;listNode.pre = last;last = listNode;}useSide++;return a;}@Overridepublic int poll() {int fisrt = peek();head = head.next;useSide --;return fisrt;}@Overridepublic int peek() {try {if(head == null){throw new EmptyException("空指针异常访问");}}catch (EmptyException e){e.printStackTrace();}return head.val;}@Overridepublic boolean isEmpty() {return useSide == 0;}}

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2.2 关于队列的例题

例题1 设计循环队列

class MyCircularQueue {public int[] array;int front;int rear;public MyCircularQueue(int k) {array = new int[k+1];}// 向循环队列插入一个元素。如果成功插入则返回真。public boolean enQueue(int value) {if(isFull()){return false;}array[rear] = value;rear = (rear+1) % array.length;return true;}//从循环队列中删除一个元素。如果成功删除则返回真。public boolean deQueue() {if(isEmpty()){return false;}front = (front+1) % array.length;return true;}//从队首获取元素。如果队列为空，返回 -1 。public int Front() {if(isEmpty()){return -1;}return array[front];}//获取队尾元素。如果队列为空，返回 -1 。public int Rear() {if(isEmpty()){return -1;}if(rear == 0){return array[array.length - 1];}return array[rear-1];}//检查循环队列是否为空。public boolean isEmpty() {return rear == front;}//检查循环队列是否已满。public boolean isFull() {return (rear+1) % array.length == front;}
}

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例题2 用队列实现栈

class MyStack {public Queue<Integer> queue1;public Queue<Integer> queue2;int size;public MyStack() {queue1 = new LinkedList<>();queue2 = new LinkedList<>();}public void push(int x) {if(queue1.isEmpty() && queue2.isEmpty()){queue1.offer(x);size++;}else if(!queue1.isEmpty()){queue1.offer(x);size++;}else if(!queue2.isEmpty()){queue2.offer(x);size++;}}public int pop() {if(queue1.isEmpty() && queue2.isEmpty()){return -1;}if(queue1.isEmpty() && !queue2.isEmpty()){int cur = size - 1;while(cur != 0){queue1.offer(queue2.poll());cur--;}size--;return queue2.poll();}else if(!queue1.isEmpty() && queue2.isEmpty()){int cur = size - 1;while(cur != 0){queue2.offer(queue1.poll());cur--;}size--;return queue1.poll();}return -1;}public int top() {if(queue1.isEmpty() && queue2.isEmpty()){return -1;}if(queue1.isEmpty() && !queue2.isEmpty()){int cur = size - 1;while(cur != 0){queue1.offer(queue2.poll());cur--;}int m = queue2.peek();queue1.offer(queue2.poll());return m;}else if(!queue1.isEmpty() && queue2.isEmpty()){int cur = size - 1;while(cur != 0){queue2.offer(queue1.poll());cur--;}int m = queue1.peek();queue2.offer(queue1.poll());return m;}return -1;}public boolean empty() {return size == 0;}}

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例题3 用栈实现队列

Stack<Integer> stack1;Stack<Integer> stack2;int size = 0;public MyQueue() {stack1 = new Stack<>();stack2 = new Stack<>();}//将元素 x 推到队列的末尾public void push(int x) {stack1.push(x);size++;}//从队列的开头移除并返回元素public int pop() {if(empty()){return -1;}if(!stack2.isEmpty()){size--;return stack2.pop();}else if(stack2.isEmpty() && !stack1.isEmpty()){while(!stack1.isEmpty()){stack2.push(stack1.pop());}size--;return stack2.pop();}return -1;}//返回队列开头的元素public int peek() {if(empty()){return -1;}if(!stack2.isEmpty()){return stack2.peek();}else if(stack2.isEmpty() && !stack1.isEmpty()){while(!stack1.isEmpty()){stack2.push(stack1.pop());}return stack2.peek();}return -1;}//如果队列为空，返回 true ；否则，返回 falsepublic boolean empty() {return size == 0;}

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总结

本篇文章介绍了有关数据结构中的栈和队列的相关内容，以及它们的简单实现，和简单使用，如果有什么不正确的或者不严谨的地方，还望指正，谢谢大家！