数组旋转k步

　　时间复杂度O(n^2) 

const rorateKstep = (arr = [1,2,3,4,5,6,7],step = 3) => {
    for(let i = 0; i < 3;i++){
        arr.unshift(arr.pop())
    }
    return arr
}

　　时间复杂度O(1)

const rorateKstep = (arr = [1,2,3,4,5,6,7],step = 3) => {
    arr = arr.splice(-step, step).concat(arr);
    return arr
}