Palindrome Pairs
<p>You are given a <strong>0-indexed</strong> array of <strong>unique</strong> strings <code>words</code>.</p>

<p>A <strong>palindrome pair</strong> is a pair of integers <code>(i, j)</code> such that:</p>

<ul>
	<li><code>0 &lt;= i, j &lt; words.length</code>,</li>
	<li><code>i != j</code>, and</li>
	<li><code>words[i] + words[j]</code> (the concatenation of the two strings) is a <span data-keyword="palindrome-string">palindrome</span>.</li>
</ul>

<p>Return <em>an array of all the <strong>palindrome pairs</strong> of </em><code>words</code>.</p>

<p>You must write an algorithm with&nbsp;<code>O(sum of words[i].length)</code>&nbsp;runtime complexity.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;abcd&quot;,&quot;dcba&quot;,&quot;lls&quot;,&quot;s&quot;,&quot;sssll&quot;]
<strong>Output:</strong> [[0,1],[1,0],[3,2],[2,4]]
<strong>Explanation:</strong> The palindromes are [&quot;abcddcba&quot;,&quot;dcbaabcd&quot;,&quot;slls&quot;,&quot;llssssll&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;bat&quot;,&quot;tab&quot;,&quot;cat&quot;]
<strong>Output:</strong> [[0,1],[1,0]]
<strong>Explanation:</strong> The palindromes are [&quot;battab&quot;,&quot;tabbat&quot;]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;a&quot;,&quot;&quot;]
<strong>Output:</strong> [[0,1],[1,0]]
<strong>Explanation:</strong> The palindromes are [&quot;a&quot;,&quot;a&quot;]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= words.length &lt;= 5000</code></li>
	<li><code>0 &lt;= words[i].length &lt;= 300</code></li>
	<li><code>words[i]</code> consists of lowercase English letters.</li>
</ul>
Trie; Array; Hash Table; String
using System.Collections.Generic;
using System.Linq;

public class Solution {
    public IList<IList<int>> PalindromePairs(string[] words) {
        var results = new List<IList<int>>();
        var reverseDict = words.Select((w, i) => new {Word = w, Index = i}).ToDictionary(w => new string(w.Word.Reverse().ToArray()), w => w.Index);

        for (var i = 0; i < words.Length; ++i)
        {
            var word = words[i];
            for (var j = 0; j <= word.Length; ++j)
            {
                if (j > 0 && IsPalindrome(word, 0, j - 1))
                {
                    var suffix = word.Substring(j);
                    int pairIndex;
                    if (reverseDict.TryGetValue(suffix, out pairIndex) && i != pairIndex)
                    {
                        results.Add(new [] { pairIndex, i});
                    }
                }
                if (IsPalindrome(word, j, word.Length - 1))
                {
                    var prefix = word.Substring(0, j);
                    int pairIndex;
                    if (reverseDict.TryGetValue(prefix, out pairIndex) && i != pairIndex)
                    {
                        results.Add(new [] { i, pairIndex});
                    }
                }
            }
        }

        return results;
    }

    private bool IsPalindrome(string word, int startIndex, int endIndex)
    {
        var i = startIndex;
        var j = endIndex;
        while (i < j)
        {
            if (word[i] != word[j]) return false;
            ++i;
            --j;
        }
        return true;
    }
}