[LeetCode]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).

［思路］利用两个堆栈实现队列结构。最容易想到的，push时，push入一个堆栈中。pop时，把堆栈内所有元素弹出到另一个堆栈中（利用两次后入先出实现先入先出），再把元素弹出。特别注意一点，当第二个堆栈不是全空的时候，pop出堆栈顶即可；全空的时候要把第一个堆栈中元素全部弹出到另一个堆栈。不必把第二个堆栈中元素全部弹回第一个堆栈。

class Queue {
private:
stack<int> stack1;
stack<int> stack2;
public:
// Push element x to the back of queue.
void push(int x) {
stack1.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if(!stack2.empty()){
stack2.pop();
}
else{
while(!stack1.empty()){
int temp = stack1.top();
stack1.pop();
stack2.push(temp);
}
stack2.pop();
}

}

// Get the front element.
int peek(void) {
if(!stack2.empty()){
return stack2.top();
}
else{
while(!stack1.empty()){
int temp = stack1.top();
stack1.pop();
stack2.push(temp);
}
return stack2.top();
}
}

// Return whether the queue is empty.
bool empty(void) {
return stack1.empty()&&stack2.empty();
}
};