225. Implement Stack using Queues

Title

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).

solution

class Stack {public:    // Push element x onto stack.    void push(int x) {        s1.push(x);    }    // Removes the element on top of the stack.    void pop() {        while(s1.size() > 1) {            s2.push(s1.front());            s1.pop();        }        if(s1.size() == 1) {            s1.pop();        }        while(!s2.empty()) {            s1.push(s2.front());            s2.pop();        }    }    // Get the top element.    int top() {        while(s1.size() > 1) {            s2.push(s1.front());            s1.pop();        }        if(s1.size() == 1) {            t = s1.front();            s2.push(s1.front());            s1.pop();        }        while(!s2.empty()) {            s1.push(s2.front());            s2.pop();        }        return t;    }    // Return whether the stack is empty.    bool empty() {        return s1.empty();    }private:    queue <int> s1, s2;    int t;};

run time: 0ms

两个队列