[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).

两个queue实现stack。

q1移入q2,留下一个，就是top和pop的那个数了。

class Stack {
public:
    queue<int> q1,q2;
    // Push element x onto stack.
    void push(int x) {
        q1.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        while(q1.size()>1)
        {
            int tmp = q1.front();
            q1.pop();
            q2.push(tmp);
        }
        q1.pop();
        while(q2.size()>0)
        {
            int tmp = q2.front();
            q2.pop();
            q1.push(tmp);
        }
    }

    // Get the top element.
    int top() {
        while(q1.size()>1)
        {
            int tmp = q1.front();
            q1.pop();
            q2.push(tmp);
        }
        return q1.front();
    }

    // Return whether the stack is empty.
    bool empty() {
        if(q1.empty() && q2.empty()) return true;
        else return false;
    }
};