Implement Queue using Stacks

 Implement Queue using Stacks

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).

解析：
利用栈实现队列的功能，使用两个栈，一个栈用来进元素，一个栈用来出元素，只有当出元素的栈为空时，把进元素栈中的所有元素放到出元素的栈中。

代码：

class MyQueue {
public:
/** Initialize your data structure here. */
stack<int>push_stack;
stack<int>pop_stack;
MyQueue() {

}

/** Push element x to the back of queue. */
void push(int x) {
push_stack.push(x);
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
if (!pop_stack.empty())
{
int temp=pop_stack.top();
pop_stack.pop();
return temp;
}
else
{
while(!push_stack.empty())
{
int temp=push_stack.top();
push_stack.pop();
pop_stack.push(temp);
}
int t=pop_stack.top();
pop_stack.pop();
return t;

}
}

/** Get the front element. */
int peek() {
if (!pop_stack.empty())
{
int temp=pop_stack.top();
return temp;
}
else
{
while(!push_stack.empty())
{
int
a7a6
temp=push_stack.top();
push_stack.pop();
pop_stack.push(temp);
}
int t=pop_stack.top();
return t;

}
}

/** Returns whether the queue is empty. */
bool empty() {
if (pop_stack.empty()&&push_stack.empty())
{
return true;
}
return false;
}
};

/**
* 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();
*/