LeetCode：Implement Queue using Stacks

LeetCode：Implement Queue using Stacks

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).

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;
};

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。