## # 232. 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.

Example:

``````MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false
``````

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).

### # Solution

Approach 1: Two Stacks: Push - O(n) per operation, Pop - O(1) per operation.

Approach 2: Two Stacks: Push - O(1) per operation, Pop - Amortized O(1) per.

Approach 1:

Approach 2:

### # Code (C++)

Approach 1:

``````/**
* 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();
*/
class MyQueue {
private:
stack<int> s1;
stack<int> s2;
public:
// Initialize your data structure here.
MyQueue() {
}

// Push element x to the back of queue.
void push(int x) {
while (!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
s1.push(x);
while (!s2.empty())
{
s1.push(s2.top());
s2.pop();
}
}

// Removes the element from in front of queue and returns that element.
int pop() {
int popVal = s1.top();
s1.pop();
return popVal;
}

// Get the front element.
int peek() {
return s1.top();
}

// Returns whether the queue is empty.
bool empty() {
return s1.empty();
}
};
``````

Approach 2:

``````class MyQueue {
private:
stack<int> s1;
stack<int> s2;
int front;
public:
// Initialize your data structure here.
MyQueue() {
}

// Push element x to the back of queue.
void push(int x) {
if (s1.empty())
front = x;
s1.push(x);
}

// Removes the element from in front of queue and returns that element.
int pop() {
if (s2.empty())
{
while (!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
}
int popVal = s2.top();
s2.pop();
return popVal;
}

// Get the front element.
int peek() {
if (!s2.empty())
return s2.top();
return front;
}

// Returns whether the queue is empty.
bool empty() {
return s1.empty() && s2.empty();
}
};
``````