Leetcode 225. Implement Stack using Queues

原題：

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).

解決方法：
關鍵在於push方法，每次加入新的數後，將原來的數從queue中彈出，並依次加入到queue中。新的數就變成第一個數。

代碼：

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