LeetCode: 232. Implement Queue using Stacks

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题目描述

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:

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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).

使用stack模拟queue，对于peek和pop操作需要另外一个stack协助完成，原stack进行pop操作的时候另一个stack进行push来保存所有元素。

代码实现

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class MyQueue {
private:
    stack<int> _stack;

public:
    /** Initialize your data structure here. */
    MyQueue() {

    }

    /** Push element x to the back of queue. */
    void push(int x) {
        this->_stack.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        stack<int> tmp;
        assert (!this->_stack.empty());
        int popNum = 0;
        while (!this->_stack.empty()) {
            popNum = this->_stack.top();
            this->_stack.pop();
            if (!this->_stack.empty()) {
                tmp.push(popNum);
            }
        }
        while (!tmp.empty()) {
            this->_stack.push(tmp.top());
            tmp.pop();
        }
        return popNum;
    }

    /** Get the front element. */
    int peek() {
        stack<int> tmp;
        int topNum = 0;
        while (!this->_stack.empty()) {
            tmp.push(this->_stack.top());
            this->_stack.pop();
        }
        topNum = tmp.top();
        while (!tmp.empty()) {
            this->push(tmp.top());
            tmp.pop();
        }
        return topNum;
    }

    /** Returns whether the queue is empty. */
    bool empty() {
        return this->_stack.empty();
    }
};