LeetCode：Implement Queue using Stacks

LeetCode：Implement Queue using Stacks

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1、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.
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).

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2、代码：

class Queue {public:    // Push element x to the back of queue.    void push(int x) {       spush.push(x);     }    // Removes the element from in front of queue.    void pop(void) {       if(spop.empty())       {           while(!spush.empty())           {               spop.push(spush.top());               spush.pop();           }           if(!spop.empty())           {               spop.pop();           }           else           {               cerr<<"no element"<<endl;           }       }       else       {          spop.pop();        }    }    // Get the front element.    int peek(void) {       if(spop.empty())       {           while(!spush.empty())           {               spop.push(spush.top());               spush.pop();           }           if(!spop.empty())           {               return spop.top();           }           else           {               cerr<<"no element"<<endl;               return -1;           }       }       else       {          return spop.top();        }     }    // Return whether the queue is empty.    bool empty(void) {        return spop.empty()&&spush.empty();    }private:    stack<int> spush,spop;};

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3、总结：
A、思路：两个栈，一个用于push，一个用于pop，当发现pop栈没有内容时就从push栈将元素全部弹出压入push栈。
B、栈的使用（C++）：

C++ STL–stack/queue 的使用方法
http://www.cnblogs.com/mfryf/archive/2012/08/09/2629992.html

stack 模板类的定义在头文件中。
stack 模板类需要两个模板参数，一个是元素类型，一个容器类型，但只有元素类型是必要
的，在不指定容器类型时，默认的容器类型为deque。
定义stack 对象的示例代码如下：
stack s1;
stack s2;
stack 的基本操作有：
入栈，如例：s.push(x);
出栈，如例：s.pop();注意，出栈操作只是删除栈顶元素，并不返回该元素。
访问栈顶，如例：s.top()
判断栈空，如例：s.empty()，当栈空时，返回true。
访问栈中的元素个数，如例：s.size()。

D、注意出栈，并不返回元素，这个好像是C++一贯风格。

E、至于stack元素类型和容器类型确定。默认deque，可以根据自己需求设定。