第二个方法，是Code Granker上面的，利用了高中学排列组合时候的概念。 有个‘abc’，那么总共可以看成'a_b_c' 5个字符位置 ...

// Find the longest palindromic substring using expand-around-center approach. // For each position, try expanding outward while characters match. // Check both odd-length (single center) and ...