Given a list of unique words, return all the pairs of the distinct indices `(i, j)` in the given list, so that the concatenation of the two words `words[i] + words[j]` is a palindrome.

Example 1:

    Input: words = ["abcd","dcba","lls","s","sssll"]
    Output: [[0,1],[1,0],[3,2],[2,4]]
    Explanation: The palindromes are ["dcbaabcd","abcddcba","slls","llssssll"]

Example 2:

    Input: words = ["bat","tab","cat"]
    Output: [[0,1],[1,0]]
    Explanation: The palindromes are ["battab","tabbat"]

Example 3:

    Input: words = ["a",""]
    Output: [[0,1],[1,0]]

Constraints:

- `1 <= words.length <= 5000`
- `0 <= words[i].length <= 300`
- `words[i]` consists of lower-case English letters.