请用栈实现一个队列,支持如下四种操作:
- push(x) – 将元素x插到队尾;
- pop() – 将队首的元素弹出,并返回该元素;
- peek() – 返回队首元素;
- empty() – 返回队列是否为空;
注意:
- 你只能使用栈的标准操作:push to top,peek/pop from top, size 和 is empty;
- 如果你选择的编程语言没有栈的标准库,你可以使用list或者deque等模拟栈的操作;
- 输入数据保证合法,例如,在队列为空时,不会进行pop或者peek等操作; 样例
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false
class MyQueue {
public:
/** Initialize your data structure here. */
stack<int> stk, cache;
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
stk.push(x);
}
void copy(stack<int> &a, stack<int> &b) {
while (a.size()) {
b.push(a.top());
a.pop();
}
}
/** 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();
*/