2131. Longest Palindrome by Concatenating Two Letter Words

T: O(n)
S: O(26*26)

```java
class Solution {
    public int longestPalindrome(String[] words) {
        int counter[][] = new int[26][26];
        int result = 0;
        for (String w : words) {
            int a = w.charAt(0) - 'a';
            int b = w.charAt(1) - 'a';
            if (counter[b][a] > 0) { // find pair! add 4
                result += 4;
                counter[b][a]--; // remove this one
            } else {
                counter[a][b]++;
            }
        }
        for (int i = 0; i < 26; i++) {
            if (counter[i][i] > 0) { // only one same can count to 2
                result += 2;
                break;
            }
        }
        return result;
    }
}

/**


T: O(n)
S: O(26*26)


["dd","aa","bb","dd","aa","dd","bb","dd","aa","cc","bb","cc","dd","cc"]

cc 0     4                                                          x
bb 0     4
aa 1     4 
dd 1.    8

20 + 2
 */
```