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

    // Removes the element from in front of queue.
    void pop(void) {
        
        if(stackB.empty())
        {
            while(!stackA.empty())
            {
                stackB.push(stackA.top());
                stackA.pop();
            }
        } 
        if(!stackB.empty())
            stackB.pop();
    }

    // Get the front element. 一般为front
    int peek(void) {
        
        if(stackB.empty())
        {
            while(!stackA.empty())
            {
                stackB.push(stackA.top());
                stackA.pop();
            }
        }
        if(!stackB.empty())
        {
            return stackB.top();
           // stackB.pop();
        }
    }

    // Return whether the queue is empty.
    bool empty(void) {
        
        if(stackB.empty()&&stackA.empty())
            return true;
        else
            return false;
    }
    
    stack<int> stackA;
    stack<int> stackB;
};

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