225.ImplementStackusingQueues.md

225. Implement Stack using Queues

• 225. Implement Stack using Queues [Stack] [Design] [Easy]

Tags

• [Stack] [Design] [Easy]

Problem

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).

Solution

class MyStack {
private:
    queue<int> q1, q2;
    
public:
    /** Push element x onto stack. */
    void push(int x) {
        q2.push(x);
        while (!q1.empty()) {
            q2.push(q1.front());
            q1.pop();
        }
        swap(q1, q2);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int result = q1.front();
        q1.pop();
        return result;
    }
    
    /** Get the top element. */
    int top() {
        return q1.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty();
    }
};

Notes

• Use Queue, 20190217.

Related Problems

• 225. Implement Stack using Queues [Stack] [Design] [Easy]