【LeetCode 0232】【设计】用FILO栈实现FIFO队列

232. Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:* void push(int x) Pushes element x to the back of the queue.* int pop() Removes the element from the front of the queue and returns it.* int peek() Returns the element at the front of the queue.* boolean empty() Returns true if the queue is empty, false otherwise.

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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.

Example 1:

**Input**
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
**Output**
[null, null, null, 1, 1, false]

**Explanation**
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Constraints:* 1 <= x <= 9* At most 100 calls will be made to push, pop, peek, and empty.* All the calls to pop and peek are valid.

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

JavaScript Solution

var MyQueue = function() {
    this.inputStack = []
    this.outputStack = []
};

/** 
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function(x) {
    this.inputStack.push(x)
};

/**
 * @return {number}
 */
MyQueue.prototype.pop = function() {
    if(!this.outputStack.length){
        while(this.inputStack.length){
            this.outputStack.push(this.inputStack.pop())
        }
    }
    if(!this.outputStack.length){
        return null
    }
    return this.outputStack.pop()
};

/**
 * @return {number}
 */
MyQueue.prototype.peek = function() {
    if(!this.outputStack.length){
        while(this.inputStack.length){
            this.outputStack.push(this.inputStack.pop())
        }
    }
    if(!this.outputStack.length){
        return null
    }
    return this.outputStack[this.outputStack.length-1]
};

/**
 * @return {boolean}
 */
MyQueue.prototype.empty = function() {
    return !this.inputStack.length && !this.outputStack.length
};

/** 
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */