1. Description

Implement Stack using Queues

2. Solution

Version 1

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

/**
 * 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();
 */

Version 2

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

/**
 * 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();
 */