目录

225. 用队列实现栈

232. 用栈实现队列

387. 字符串中的第一个唯一字符

622. 设计循环队列

641. 设计循环双端队列

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225. 用队列实现栈

https://leetcode.cn/problems/implement-stack-using-queues/

class MyStack{private Queue<Integer> queue1;private Queue<Integer> queue2;//构造方法；public MyStack() {queue1 = new LinkedList<>();queue2 = new LinkedList<>();}//压栈；public void push(int x){//两个队列为空，则选一个先入队列，这里选queue2；//之后哪个队列非空，则入哪个队列；if (!queue1.isEmpty()){queue1.offer(x);}else{queue2.offer(x);}}//出栈；public int pop(){//哪个队列非空，就将这个队列的元素转移到另一个队列；//转移过程中，记录每一个出队列的元素；//直到出元素的队列为空，不将该元素入队列，而是返回；if(!queue1.isEmpty()){int temp = queue1.poll();while (!queue1.isEmpty()){queue2.offer(temp);temp = queue1.poll();}return temp;}else if(!queue2.isEmpty()){int temp = queue2.poll();while (!queue2.isEmpty()){queue1.offer(temp);temp = queue2.poll();}return temp;}return -1;}//查看栈顶元素；public int top(){//哪个队列非空，就将这个队列的元素转移到另一个队列；//转移过程中，记录每一个出队列的元素，直到出元素的队列为空，返回记录的元素；int temp = -1;if(!queue1.isEmpty()){while(!queue1.isEmpty()){temp = queue1.poll();queue2.offer(temp);}}else if(!queue2.isEmpty()){while(!queue2.isEmpty()){temp = queue2.poll();queue1.offer(temp);}}return temp;}//判断栈是否为空；public boolean empty(){if(queue1.isEmpty() && queue2.isEmpty()){return true;}else {return false;}}
}

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232. 用栈实现队列

https://leetcode.cn/problems/implement-queue-using-stacks/

class MyQueue{//两个栈；private Stack<Integer> stack1 = null;private Stack<Integer> stack2 = null;//构造方法，初始化两个栈；public MyQueue() {stack1 = new Stack<>();stack2 = new Stack<>();}//压栈，stack1负责元素入队列；public void push(int x){stack1.push(x);}//stack2负责元素出队列；public int pop(){//如果stack2不为空，直接出；if(!stack2.isEmpty()){return stack2.pop();}else {//如果stack2为空，看看stack1是否为空；//不为空将stack1元素转移到stack2，再将stack2栈顶元素出栈；if(!stack1.isEmpty()){while (!stack1.isEmpty()){stack2.push(stack1.pop());}return stack2.pop();}}return -1;}//stack2的栈顶元素就是队列的队首元素；public int peek(){//如果stack2不为空，返回栈顶元素；if(!stack2.isEmpty()){return stack2.peek();}else {//如果stack2为空，看看stack1是否为空；//不为空将stack1元素转移到stack2，再将stack2栈顶元素peek；if(!stack1.isEmpty()){while (!stack1.isEmpty()){stack2.push(stack1.pop());}return stack2.peek();}}return -1;}public boolean empty(){return stack1.isEmpty() && stack2.isEmpty();}
}

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387. 字符串中的第一个唯一字符

https://leetcode.cn/problems/first-unique-character-in-a-string/

    public int firstUniqChar1(String s) {//创建一个代表26个英文字母的数组；int[] arr = new int[26];//将字符串s转换为字符数组；(也可以不转，使用String的方法即可)char[] chs = s.toCharArray();//将每个字符转换成整数，这个整数代表下标，放入arr数组中；for(int i=0;i<chs.length;i++){int n = chs[i] - 'a';arr[n]++;}//遍历字符数组，查arr数组元素，找到第一个元素为1的，就返回这个字符；for(int i=0;i<chs.length;i++){int n = chs[i] - 'a';if(arr[n] == 1){return i;}}//找不到返回-1；return -1;}

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622. 设计循环队列

https://leetcode.cn/problems/design-circular-queue/

class MyCircularQueue {//用于存放元素的数组elem，队首下标front，队尾下标rear，队列有效元素size；private int[] elem;private int front;private int rear;private int size;//构造方法，构造容量为k的数组；public MyCircularQueue(int k) {elem = new int[k];}//入队列方法；public boolean enQueue(int value) {if(this.isFull()){return false;}//赋值给队尾元素，rear++；elem[rear] = value;rear++;//超过最大容量，下标回到0；if(rear >= elem.length){rear = 0;}//有效元素++；size++;return true;}//出队列方法；public boolean deQueue() {if(this.isEmpty()){return false;}//队首下标直接向后移动；front++;//超过最大容量，下标回到0；if(front >= elem.length){front = 0;}//有效元素--；size--;return true;}//查看队首元素；public int Front() {if(this.isEmpty()){return -1;}return elem[front];}//查看队尾元素；public int Rear() {if(this.isEmpty()){return -1;}//应注意队尾在每一次入队列后都会向后移动；//因此想得到队尾元素需要查看队尾下标的前一个元素；int temp = rear-1;//处理循环队列中0下标的转换问题；if(rear == 0){temp = elem.length-1;}return elem[temp];}//有效元素为0，则队列为空；public boolean isEmpty() {return size == 0;}//有效元素等于数组容量，则队列满；public boolean isFull() {return size == elem.length;}
}

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641. 设计循环双端队列

https://leetcode.cn/problems/design-circular-deque/

class MyCircularDeque {//用于存放元素的数组，队首元素下标，队尾元素下标，有效元素个数；private int[] elems;private int front;private int rear;private int size;//构造方法；public MyCircularDeque(int k) {elems = new int[k];}//头插法；public boolean insertFront(int value) {//判断是否已满；if(this.isFull()){return false;}//队首指针前移，注意转换最大下标和0下标；if(front == 0){front = elems.length-1;}else{front--;}//赋值和有效元素++；elems[front] = value;size++;return true;}//尾插法；public boolean insertLast(int value) {//判断是否已满；if(this.isFull()){return false;}//赋值和有效元素++；elems[rear] = value;size++;//队尾指针后移，注意转换最大下标和0下标；if(rear == elems.length-1){rear = 0;}else{rear++;}return true;}//头删法；public boolean deleteFront() {//判断是否为空；if(this.isEmpty()){return false;}//队首指针后移，注意转换最大下标和0下标；if(front == elems.length-1){front = 0;}else{front++;}//有效元素--；size--;return true;}//尾删法；public boolean deleteLast() {//判断是否为空；if(this.isEmpty()){return false;}//队尾指针前移，注意转换最大下标和0下标；if(rear == 0){rear = elems.length-1;}else{rear--;}//有效元素--；size--;return true;}//查看队首元素；public int getFront() {if(this.isEmpty()){return -1;}return elems[front];}//查看队尾元素；public int getRear() {if(this.isEmpty()){return -1;}//注意队尾元素应该是队尾指针的前一个，注意转换最大下标和0下标；int temp = rear-1;if(rear == 0){temp = elems.length-1;}return elems[temp];}//判断是否为空；public boolean isEmpty() {return size == 0;}//判断是否已满；public boolean isFull() {return size == elems.length;}
}

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