225. Implement Stack using Queues

• Difficulty: Easy

• Topics: Stack, Design

• Similar Questions:

  • Implement Queue using Stacks

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).

Solutions:

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {

    }

    /** 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 val = top();
        q1.pop();
        return val;
    }

    /** Get the top element. */
    int top() {
        return q1.front();
    }

    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty();   
    }

private:
    queue<int> q1;
    queue<int> q2; 
};

/**
 * 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();
 */