[LeetCode] 232 - Implement Queue using Stacks

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 emptyoperations 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).

class Queue {
public:
　　// Push element x to the back of queue.
　　void push(int x) {
　　　　m_inStack.push(x);
　　}

　　// Removes the element from in front of queue.
　　void pop(void) {
　　　　if (m_outStack.empty()) {
　　　　　　if (m_inStack.empty()) {
　　　　　　　　return;
　　　　　　}
　　　　　　while (!m_inStack.empty()) {
　　　　　　　　int val = m_inStack.top();
　　　　　　　　m_inStack.pop();
　　　　　　　　m_outStack.emplace(val);
　　　　　　}
　　　　}
　　　　m_outStack.pop();
　　}

　　// Get the front element.
　　int peek(void) {
　　　　if (m_outStack.empty()) {
　　　　　　if (m_inStack.empty()) {
　　　　　　　　return -1;
　　　　　　}
　　　　　　while(!m_inStack.empty()) {
　　　　　　　　int val = m_inStack.top();
　　　　　　　　m_inStack.pop();
　　　　　　　　m_outStack.emplace(val);
　　　　　　}
　　　　}
　　　　return m_outStack.top();
　　}

　　// Return whether the queue is empty.
　　bool empty(void) {
　　　　return (m_inStack.empty() && m_outStack.empty());
　　}
private:
　　stack<int>m_inStack;
　　stack<int>m_outStack;
};