225. Implement Stack using Queues

1. Description

Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).
Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

2. Notes

• You must use only standard operations of a queue, which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue’s standard operations.

3. Follow Up

Can you implement the stack such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

4. Example

Example 1:
Input
[“MyStack”, “push”, “push”, “top”, “pop”, “empty”]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

5. Note

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

6. Solutions

My Accepted Solution(Follow Up)

n is count of numbers in the stack
Time complexity: O(n)
Space complexity: O(n)

// if the push operation is more frequent, we could design the stack like mine
// if the pop or top operation is more freuent, we could design the stack by letting pop and top operation to O(1), letting push operation to O(n)
// we could make this by changing the push operation, we push the value, then pop all existed value and push them back, so we could reverse the queue, make its order like a real stack

class MyStack 
{
private:
    queue<int> storage;
    queue<int> operation;
public:
    MyStack() {}

    void push(int x) 
    {
        storage.push(x);
    }
    
    int pop() 
    {
        int lastValue;
        while(storage.size() > 1)
        {
            operation.push(storage.front());
            storage.pop();
        }
        
        lastValue = storage.front();
        storage.pop();
        swap(storage, operation);
        
        return lastValue;
    }
    
    int top() 
    {
        int lastValue;
        while(!storage.empty())
        {
            lastValue = storage.front();
            operation.push(storage.front());
            storage.pop();
        }
        
        swap(storage, operation);
        
        return lastValue;
    }
    
    bool empty() 
    {
        return storage.empty();
    }
};