336. Palindrome Pairs

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.