Implement Queue using Stacks（用栈实现队列）

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).

题目解析：
用堆栈stack实现队列，只能使用堆栈的操作，push()，pop（），empty().

方法：
用两个堆栈实现。代码如下：

class Queue {
public:
stack<int> st1;
stack<int> st2;
bool st1_use=1;
bool st2_use=0;
// Push element x to the back of queue.
void push(int x) {
if(st1_use==1)
st1.push(x);
else if(st2_use==1)
st2.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if(st1.empty()&&st2.empty())
exit(0);
if(st1_use==1&&st2_use==0)
{
while(st1.size()>1)
{
int temp=st1.top();
st2.push(temp);
st1.pop();
}
st1.pop();
while(st2.size()>0)
{
int temp=st2.top();
st1.push(temp);
st2.pop();
}
st1_use==1;
st2_use==0;
return;
}
if(st1_use==0&&st2_use==1)
{
while(st2.size()>1)
{
int temp=st2.top();
st1.push(temp);
st2.pop();
}
st2.pop();
while(st1.size()>0)
{
int temp=st1.top();
st2.push(temp);
st1.pop();
}
st1_use==0;
st2_use==1;
return;
}
}

// Get the front element.
int peek(void) {
if(st1_use==1&&st2_use==0)
{
int temp;
while(st1.size()>0)
{
temp=st1.top();
st2.push(temp);
st1.pop();
}

while(st2.size()>0)
{
int temp1=st2.top();
st1.push(temp1);
st2.pop();
}
st1_use==1;
st2_use==0;
return temp;
}
if(st1_use==0&&st2_use==1)
{
int temp;
while(st2.size()>0)
{
int temp=st2.top();
st1.push(temp);
st2.pop();
}

while(st1.size()>0)
{
int temp1=st1.top();
st2.push(temp1);
st1.pop();
}
st1_use==0;
st2_use==1;
return temp;
}
}

// Return whether the queue is empty.
bool empty(void) {
if(st1_use==1&&st1.empty())
return true;
if(st2_use==1&&st2.empty())
return true;
return false;
}

};