# LeetCode – Implement Queue using Stacks
Date: 2019-09-08


### **题目：**

Implement the following operations of a queue using stacks.

- push(x) -- Push element x to the back of queue.
- pop() -- Removes the element from in front of queue.
- peek() -- Get the front element.
- empty() -- Return whether the queue is empty.

**Example:**

```
MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false
```

**Notes:**

- You must use _only_ standard operations of a stack -- which means only `push to top`, `peek/pop from top`, `size`, and `is empty` operations are valid.
- Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
- You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

### **解题：**

         本地使用stack实现一个queue的功能， queue特点是先进先出， stack的特点是先进后出

1.  创建一个辅助栈tmp，把s的元素也逆着顺序存入tmp中，此时加入新元素x，再把tmp中的元素取出放入input，其他三个操作也就直接调用栈的操作即可，参见代码如下：                  

### **实现：**

```
class MyQueue {
private:
    stack<int> input,output;
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
  /** Push element x to the back of queue. */
    void push(int x) {
        stack<int> tmp;
        while (!input.empty()){
            tmp.push(input.top());
            input.pop();
        }
        input.push(x);
        while(!tmp.empty()){
            input.push(tmp.top());
            tmp.pop();
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
       
        int val;
        val = input.top();
        input.pop();
        return val;
    }
    
    /** Get the front element. */
    int peek() {
       return input.top();
        
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return input.empty();
        
    }
    
```

### **解题：**

2 .入栈不处理， 出栈的时候，通过临时栈进行转换

```
class MyQueue {
private:
    stack<int> input,output;
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }

    /** Push element x to the back of queue. */
    void push(int x) {
        input.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        peek();
        int val;
        val = output.top();
        output.pop();
        return val;
    }
    
    /** Get the front element. */
    int peek() {
        if (output.empty()) {
           while(!input.empty()) {
               output.push(input.top());
               input.pop();
           }
        }

        return output.top();
        
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return input.empty() && output.empty();
        
    }
}
```