nums = "----->-->"; k =3
result = "-->----->";

reverse "----->-->" we can get "<--<-----"
reverse "<--" we can get "--><-----"
reverse "<-----" we can get "-->----->"

参考：
https://leetcode-cn.com/problems/rotate-array/solution/xuan-zhuan-shu-zu-by-leetcode-solution-nipk/