336. Palindrome Pairs
Hard
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431

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
.

Return an array of all the palindrome pairs of `words`.

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.
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