剑指offer：面试题09. 用两个栈实现队列

题目：用两个栈实现队列

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )

示例 1：

输入：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
输出：[null,null,3,-1]

示例 2：

输入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

提示：

1 <= values <= 10000
最多会对 appendTail、deleteHead 进行 10000 次调用

---

解题：

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    stack<int> stk, cache;
    /** Push element x to the back of queue. */
    void copy(stack<int>& a, stack<int>& b){
        while(a.size()){
            b.push(a.top());
            a.pop();
        }
    }
    void push(int x) {
        stk.push(x);
    }
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        copy(stk, cache);
        int res = cache.top();
        cache.pop();
        copy(cache, stk);
        return res;
    }
    
    /** Get the front element. */
    int peek() {
        copy(stk, cache);
        int res = cache.top();
        copy(cache, stk);
        return res;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return stk.empty();
    }
};

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