【LeetCode】232. Implement Queue using Stacks

class Queue {
public:
    stack<int> stk1;    // push
    stack<int> stk2;    // pop
    
    // Push element x to the back of queue.
    void push(int x) {
        stk1.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if(stk2.empty())
        {
            while(!stk1.empty())
            {
                int top = stk1.top();
                stk1.pop();
                stk2.push(top);
            }
        }
        stk2.pop();
    }

    // Get the front element.
    int peek(void) {
        if(stk2.empty())
        {
            while(!stk1.empty())
            {
                int top = stk1.top();
                stk1.pop();
                stk2.push(top);
            }
        }
        return stk2.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return stk1.empty()&&stk2.empty();
    }
};