Implement Queue using Stacks

该题算是比较简单的，思路是定义两个栈，一个做为入栈，一个做为出栈。入队列的话直接将元素压入入栈，出队列的话首先判断出栈是否为空，为空则将入栈中的元素依次压入出栈中，再从出栈弹出元素。

class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
instack.push(x);

}
// Removes the element from in front of queue.
void pop(void) {
if(!outstack.empty()){
outstack.pop();
return;
}
if(!instack.empty()){
while(!instack.empty()){
outstack.push(instack.top());
instack.pop();
}
outstack.pop();
}
}
// Get the front element.
int peek(void) {
if(!outstack.empty()){
return outstack.top();
}
if(!instack.empty()){
while(!instack.empty()){
outstack.push(instack.top());
instack.pop();
}
return outstack.top();
}
}
// Return whether the queue is empty.
bool empty(void) {
return !instack.empty() || !outstack.empty() ? false : true;

}
private: 

//辅助栈
stack<int> instack, outstack;
};