LEETCODE-Implement Queue using Stacks

最新推荐文章于 2022-04-18 21:48:00 发布

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本文介绍了一种使用两个栈来实现队列的方法。通过将入队操作映射为向一个栈中压入元素，出队操作映射为从另一个栈中弹出元素的方式，实现了队列的基本操作：push、pop、peek和empty。这种方法巧妙地利用了栈的特性来模拟队列的行为。

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.
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).

class Queue {
public:
    stack<int> in;
    stack<int> out;
    // Push element x to the back of queue.
    void push(int x) {
        in.push(x);
    }


    // Removes the element from in front of queue.
    void pop(void) {
        if(out.empty())
        {
            while(!in.empty())
         {
             int x = in.top();
             in.pop();
             out.push(x);
         }
        }
       // if(!out.empty())
           out.pop();
    }

    // Get the front element.
    int peek(void) {
        if(out.empty())
        {
         while(!in.empty())
         {
             int x = in.top();
             in.pop();
             out.push(x);
         }
        }
       // if(!out.empty())
          return out.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return out.empty() && in.empty();
    }
};