232. 用栈实现队列

✔做题思路 or 感想 ：

• 用栈来实现队列的一般方法是：造一个输入栈和一个输出栈来模拟队列
  • 当要push时，则把元素push进输入栈
  • 当要pop时，检测输出栈是否有元素。若有，则直接把输出栈的栈顶元素pop掉就好。若无，则要先把输入栈的元素倒进输出栈，再进行操作

class MyQueue {
public:
    MyQueue() {

    }
    stack<int> InSt;	//先造两个输入，输出栈 
    stack<int> OutSt;
    void push(int x) {
        InSt.push(x);
    }
    
    int pop() {	//若输出栈有元素，则pop，若无，则先把输入栈倒进输出栈
        if (OutSt.size() != 0) {
            int result = OutSt.top();
            OutSt.pop();
            return result;
        } else {
            while (InSt.size() != 0) {
                OutSt.push(InSt.top());
                InSt.pop();
            }
            int result = OutSt.top();
            OutSt.pop();
            return result;
        }
    }
    
    int peek() {
        if (OutSt.size() != 0) {
            int result = OutSt.top();
            return result;
        } else {
            while (InSt.size() != 0) {
                OutSt.push(InSt.top());
                InSt.pop();
            }
            int result = OutSt.top();
            return result;
        }
    }
    
    bool empty() {
        return InSt.empty() && OutSt.empty();   
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */