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by Sumit Chourasia | Oct 03, 2020 | Category :coding | Tags : algorithm data-structure easy leetcode stack

Implement Stack using Queues - Stack - Easy - LeetCode

Implement Stack using Queues - Stack - Easy - LeetCode

Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.
Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false
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, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

public class MyStack {

    Queue<int> queue1;
    Queue<int> queue2;
    /** Initialize your data structure here. */
    public MyStack() {
        queue1 = new Queue<int>();
        queue2 = new Queue<int>();
    }
    
    /** Push element x onto stack. */
    public void Push(int x) {
        queue2.Enqueue(x);
        while(queue1.Count>0){
            queue2.Enqueue(queue1.Dequeue());
        }
        Queue<int> temp = queue1;
        queue1 = queue2;
        queue2 = temp;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int Pop() {        
        if(queue1.Count<1){
            return -1;
        }
        return queue1.Dequeue();
    }
    
    /** Get the top element. */
    public int Top() {
        if(queue1.Count<1){
            return -1;
        }
        return queue1.Peek();
    }
    
    /** Returns whether the stack is empty. */
    public bool Empty() {
        return queue1.Count()<1;
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.Push(x);
 * int param_2 = obj.Pop();
 * int param_3 = obj.Top();
 * bool param_4 = obj.Empty();
 */

Time Complexity: O(n)

Space Complexity: O(n)