C : beats 100%, 4-lines, simple n short, no built-in Utils/Data Structures, O(1)S O(n)T
NOTE : Whoever downvoted this, it is clear that you failed to understand the code here since it requires a seasoned C programmer to appreciate it. So, if you do
**NOTE :** Whoever downvoted this, it is clear that you ***failed*** to understand the code here since it requires a seasoned C programmer to appreciate it. So, if you ***don\'t*** understand, move on, but do \\****not***\\* just downvote to show your frustration. Period.\n\n```\nint longestPalindrome(char * s){\n    int m[128] = { 0 }, a = 0, b = 0, c;\n    while (*s && ++m[*s++]);\n    for (int i = \'A\' - 1 ; ++i <= \'z\' ; m[i] && (a += m[i] - ((b |= c = m[i] & 0x1), c)));\n    return a + b;\n}\n```
['C']