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).

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

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

// Get the front element.
int peek(void) {
if(out.empty())
{
while(!in.empty())
{
int x = in.top();
in.pop();
out.push(x);
}
}
// if(!out.empty())
return out.top();
}

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