题目描述
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
算法1
(暴力实现)
时间复杂度
参考文献
JAVA代码
class MyQueue {
Stack<Integer> stack = new Stack<>();
Stack<Integer> cache = new Stack<>();
/** Initialize your data structure here. */
public MyQueue() {
}
public void copy(Stack<Integer> a, Stack<Integer> b){
//a倒去b
while(!a.empty()){
b.push(a.pop());
}
}
/** Push element x to the back of queue. */
public void push(int x) {
stack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
copy(stack, cache);
int top = cache.peek();
cache.pop();
copy(cache, stack);
return top;
}
/** Get the front element. */
public int peek() {
copy(stack, cache);
int top = cache.peek();
copy(cache, stack);
return top;
}
/** Returns whether the queue is empty. */
public boolean empty() {
return stack.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();
* boolean param_4 = obj.empty();
*/