• 232. Implement Queue using Stacks


    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 toppeek/pop from topsize, 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).
    class MyQueue {
    
        Stack<Integer> s1;
        Stack<Integer> s2;
        /** Initialize your data structure here. */
        public MyQueue() {
            s1 = new Stack();
            s2 = new Stack();
        }
        
        /** Push element x to the back of queue. */
        public void push(int x) {
            s1.push(x);
        }
        
        /** Removes the element from in front of queue and returns that element. */
        public int pop() {
            peek();
            return s2.pop();
        }
        
        /** Get the front element. */
        public int peek() {
            if(s2.isEmpty()) {
                while(!s1.isEmpty()) {
                    s2.push(s1.pop());
                }
            }
            return s2.peek();
        }
        
        /** Returns whether the queue is empty. */
        public boolean empty() {
            return s1.isEmpty() && s2.isEmpty();
        }
    }
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  • 原文地址:https://www.cnblogs.com/wentiliangkaihua/p/11416616.html
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