You are given a 0-indexed array of unique strings words.

A palindrome pair is a pair of integers (i, j) such that:

0 <= i, j < words.length,
i != j, and
words[i] + words[j] (the concatenation of the two strings) is a palindrome.

Return an array of all the palindrome pairs of words.

You must write an algorithm with O(sum of words[i].length) runtime complexity.

  Example 1:

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

  Constraints:

1 <= words.length <= 5000
0 <= words[i].length <= 300
words[i] consists of lowercase English letters.