225. Implement Stack using Queues

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).

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        queue<int> tmp;
        while (!q.empty()) {
            tmp.push(q.front());
            q.pop();
        }
        q.push(x);
        while (!tmp.empty()) {
            q.push(tmp.front());
            tmp.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val = q.front(); q.pop();
        return val;
    }
    
    /** Get the top element. */
    int top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
private:
    queue<int> q;
};

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for (int i = 0; i < (int)q.size() - 1; ++i) {
            q.push(q.front());
            q.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val = q.front(); q.pop();
        return val;
    }
    
    /** Get the top element. */
    int top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
private:
    queue<int> q;
};

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q2.push(x);
        while (q2.size() > 1) {
            q1.push(q2.front()); q2.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int val = top();
        q2.pop();
        return val;
    }
    
    /** Get the top element. */
    int top() {
        if (q2.empty()) {
            for (int i = 0; i < (int)q1.size() - 1; ++i) {
                q1.push(q1.front()); q1.pop();
            }
            q2.push(q1.front()); q1.pop();
        }
        return q2.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty() && q2.empty();
    }
private:
    queue<int> q1, q2;
};