LeetCode 225. Implement Stack using Queues

原题链接在这里：https://leetcode.com/problems/implement-stack-using-queues/

题目：

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题解：

与Implement Queue using Stacks相对应。

用一个queue来实现stack. push时从queue尾add进去, 然后rotate了que.size()-1次.

pop()时从queue首 poll 出一个. top()是 peek() 一下.

Time Complexity: push, O(n). pop, O(1). peek, O(1). empty, O(1).

Space: O(n), queue size.

AC java:

public class MyStack {
    
    Queue<Integer> que;
    /** Initialize your data structure here. */
    public MyStack() {
        que = new LinkedList<Integer>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        que.offer(x);
        for(int i = 0; i<que.size()-1; i++){
            que.offer(que.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return que.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return que.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return que.isEmpty();
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */

AC C++:

class MyStack {
public:
    queue<int> que;
    MyStack() {
    }
    
    void push(int x) {
        que.push(x);
        for(int i = 0; i < que.size() - 1; i++){
            que.push(que.front());
            que.pop();
        }
    }
    
    int pop() {
        int temp = que.front();
        que.pop();
        return temp;
    }
    
    int top() {
        return que.front();
    }
    
    bool empty() {
        return que.empty();
    }
};

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