Leetcode: Implement Queue using Stacks
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Implement the following operations of a queue using stacks.- push(x) -- Push element x to the back of queue.
- pop() -- Removes the element from in front of queue.
- peek() -- Get the front element.
- empty() -- Return whether the queue is empty.
- You must use only standard operations of a stack -- which means only
push to top
,peek/pop from top
,size
, andis empty
operations are valid. - Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
- You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
双堆栈模拟队列。
class Queue {public: // Push element x to the back of queue. void push(int x) { m_secondStack.push(x); } // Removes the element from in front of queue. void pop(void) { if (m_firstStack.empty()) { while (!m_secondStack.empty()) { m_firstStack.push(m_secondStack.top()); m_secondStack.pop(); } } m_firstStack.pop(); } // Get the front element. int peek(void) { if (m_firstStack.empty()) { while (!m_secondStack.empty()) { m_firstStack.push(m_secondStack.top()); m_secondStack.pop(); } } return m_firstStack.top(); } // Return whether the queue is empty. bool empty(void) { return m_firstStack.empty() && m_secondStack.empty(); } private: stack<int> m_firstStack; stack<int> m_secondStack;};
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