[leetcode] Implement Stack using Queues

From : https://leetcode.com/problems/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.

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 Stack {    queue<int> q1;    queue<int> q2;public:    // Push element x onto stack.    void push(int x) {        if(q1.empty()) q2.push(x);        else q1.push(x);    }    // Removes the element on top of the stack.    void pop() {        if(empty()) return;        if(q1.empty()) swapQueue();        while(true) {            int x = q1.front();            q1.pop();            if(q1.empty()) return;            q2.push(x);        }    }    // Get the top element.    int top() {        if(empty()) return 0;        if(q1.empty()) swapQueue();        while(true) {            int x = q1.front();            q1.pop();            q2.push(x);            if(q1.empty()) return x;        }    }    // Return whether the stack is empty.    bool empty() {        return q1.empty() && q2.empty();    }        void swapQueue() {        queue<int> t = q1;        q1 = q2;        q2 = t;    }};

class Stack {    queue<int> q;    int size=0;public:    // Push element x onto stack.    void push(int x) {        q.push(x);        size++;    }    // Removes the element on top of the stack.    void pop() {        if(size==0) return;        for(int i=1; i<size; i++) {            int x = q.front();            q.pop();            q.push(x);        }q.pop();        size--;    }    // Get the top element.    int top() {        if(size==0) return 0;        for(int i=1; i<size; i++) {            int x = q.front();            q.pop();            q.push(x);        }        int x = q.front();        q.pop();q.push(x);        return x;    }    // Return whether the stack is empty.    bool empty() {        return q.empty();    }};