LEETCODE ALGORITHM:面试题 03.04. Implement Queue using Stacks LCCI

题目

Implement a MyQueue class which implements a queue using two stacks.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // return 1
queue.pop();   // return 1
queue.empty(); // return 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).

题解

class MyQueue {
private:
    stack<int> s1,s2;
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
       if(s2.empty())
       {
           while(!s1.empty())
           {
               s2.push(s1.top());
               s1.pop();
           }
       }
       int an=s2.top();
       s2.pop();
       return an;
    }
    
    /** Get the front element. */
    int peek() {
       if(s2.empty())
       {
           while(!s1.empty())
           {
               s2.push(s1.top());
               s1.pop();
           }
       }
       int an=s2.top();
       return an;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.size()+s2.size()==0;
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */

双栈模拟队列