Given an array of string `words`. Return all strings in `words` which is substring of another word in any order.

String `words[i]` is substring of `words[j]`, if can be obtained removing some characters to left and/or right side of `words[j]`.

Example 1:

    Input: words = ["mass","as","hero","superhero"]
    Output: ["as","hero"]
    Explanation: "as" is substring of "mass" and "hero" is substring of "superhero".
    ["hero","as"] is also a valid answer.

Example 2:

    Input: words = ["leetcode","et","code"]
    Output: ["et","code"]
    Explanation: "et", "code" are substring of "leetcode".

Example 3:

    Input: words = ["blue","green","bu"]
    Output: []

Constraints:

- `1 <= words.length <= 100`
- `1 <= words[i].length <= 30`
- `words[i]` contains only lowercase English letters.
- It's guaranteed that `words[i]` will be unique.