LeetCode : 232. Implement Queue using Stacks 栈实现队列

落日映苍穹つ 2021-06-12 20:36 439阅读 0赞

**试题**  
Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue.  
pop() – Removes the element from in front of queue.  
peek() – Get the front element.  
empty() – Return whether the queue is empty.  
Example:

MyQueue queue = new MyQueue();

queue.push(1);  
queue.push(2);  
queue.peek(); // returns 1  
queue.pop(); // returns 1  
queue.empty(); // returns false  
Notes:

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

**代码**

class MyQueue {
    
        Stack<Integer> stack1, stack2;
        /** Initialize your data structure here. */
        public MyQueue() {
            stack1 = new Stack<>();
            stack2 = new Stack<>();
        }
        
        /** Push element x to the back of queue. */
        public void push(int x) {
            stack1.push(x);
        }
        
        /** Removes the element from in front of queue and returns that element. */
        public int pop() {
            if(stack2.isEmpty()){
                while(!stack1.isEmpty())
                    stack2.push(stack1.pop());
            }
            return stack2.pop();
        }
        
        /** Get the front element. */
        public int peek() {
            if(stack2.isEmpty()){
                while(!stack1.isEmpty())
                    stack2.push(stack1.pop());
            }
            return stack2.peek();
        }
        
        /** Returns whether the queue is empty. */
        public boolean empty() {
            return stack1.isEmpty() && stack2.isEmpty();
        }
    }
    
    /**
     * 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();
     */