LeetCode|Implement Stack using Queues

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 {public:    // Push element x onto stack.    void push(int x) {        if(q1.empty()){            Q1 = &q1;            Q2 = &q2;        } else {            Q1 = &q2;            Q2 = &q1;        }        Q1->push(x);        while(!Q2->empty()){            Q1->push(Q2->front());            Q2->pop();        }    }    // Removes the element on top of the stack.    void pop() {        if(q1.empty()){           q2.pop();        } else {           q1.pop();        }    }    // Get the top element.    int top() {        if(q1.empty()){           return q2.front();        } else {           return q1.front();        }    }    // Return whether the stack is empty.    bool empty() {        return q1.empty() && q2.empty();    }    queue<int> q1, q2;    queue<int> *Q1, *Q2;};

push为O(n), pop()和top()为O(1)