LeetCode: 225. Implement Stack using Queues
阿新 • • 發佈:2019-01-29
題目描述
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.
Example:
MyStack stack = new MyStack(); stack.push(1); stack.push(2); stack.top(); // returns 2 stack.pop(); // returns 2 stack.empty(); // returns false
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).
解題思路
用兩個佇列,將 push
的資料反向寫入佇列中。
AC 程式碼
class MyStack { public: /** Initialize your data structure here. */ MyStack() { } /** Push element x onto stack. */ void push(int x) { queue<int> que; while(!m_stk.empty()) { que.push(m_stk.front()); m_stk.pop(); } m_stk.push(x); while(!que.empty()) { m_stk.push(que.front()); que.pop(); } } /** Removes the element on top of the stack and returns that element. */ int pop() { int t = top(); m_stk.pop(); return t; } /** Get the top element. */ int top() { return m_stk.front(); } /** Returns whether the stack is empty. */ bool empty() { return m_stk.empty(); } private: queue<int> m_stk; }; /** * Your MyStack object will be instantiated and called as such: * MyStack obj = new MyStack(); * obj.push(x); * int param_2 = obj.pop(); * int param_3 = obj.top(); * bool param_4 = obj.empty(); */