Implement Queue using Stacks

一、题目要求

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).

二、代码实现

   // Push element x to the back of queue.    void push(int x) {        st1.push(x);    }    // Removes the element from in front of queue.    void pop(void) {        if(!st2.empty()){st2.pop();}else{while(!st1.empty()){st2.push(st1.top());st1.pop();}st2.pop();}    }    // Get the front element.    int peek(void) {         if(st2.empty()){while(!st1.empty()){st2.push(st1.top());st1.pop();}}return st2.top();    }    // Return whether the queue is empty.    bool empty(void) {        if(st1.empty()&&st2.empty())return true;return false;    }    private:stack<int> st1,st2;