概念：

数组是最基础的数据结构，特点是O(1)时间读取任意下标元素，
主要用于，排序，双指针，二分查找，动态规划
顺序访问数组，按下标取值是数组的常见操作

基础题：

https://leetcode-cn.com/problems/sort-array-by-parity/
https://leetcode-cn.com/problems/squares-of-a-sorted-array/
https://leetcode-cn.com/problems/sort-array-by-parity-ii/submissions/
https://leetcode-cn.com/problems/monotonic-array/submissions/
https://leetcode-cn.com/problems/find-all-numbers-disappeared-in-an-array/submissions/