[Leetcode 232, Easy] Implement Queue using Stacks

Problem:

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.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is 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).

Analysis:

Solutions:

C++:

class Queue {public:    // Push element x to the back of queue.    void push(int x) {        if(m_stack.empty()) {            while(!m_stack_backup.empty()) {                m_stack.push(m_stack_backup.top());                m_stack_backup.pop();            }        }        m_stack.push(x);    }    // Removes the element from in front of queue.    void pop(void) {        if(m_stack_backup.empty()) {            while(!m_stack.empty()) {                m_stack_backup.push(m_stack.top());                m_stack.pop();            }        }        m_stack_backup.pop();    }    // Get the front element.    int peek(void) {        if(m_stack_backup.empty()) {            while(!m_stack.empty()) {                m_stack_backup.push(m_stack.top());                m_stack.pop();            }        }        return m_stack_backup.top();    }    // Return whether the queue is empty.    bool empty(void) {        return (m_stack.empty() && m_stack_backup.empty());    }    private:    stack<int> m_stack;    stack<int> m_stack_backup;};

Java:

Python: