1. Diffuse from a starting point to both ends
  2. Palindrome substring can be either even or odd length
  3. Max length should be k-j+1, but when j,k exit while, j--, k++ would extend one step on both ends: k-j-1 would be the right length because of +2 difference.
  4. substring(start, end) would be a start of j+1 to justify the -1
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class Solution {
    private int begin, max;
    
    public String longestPalindrome(String s) {
        int len = s.length();
        if (len < 2) return s;
        for (int i = 0; i < s.length() - 1; ++i) {
            diffuse(s, i, i); // odd length
            diffuse(s, i, i+1); // even 
        }
        return s.substring(begin, begin + max);
    }
    
    public void diffuse(String s, int j, int k) {
        while (j >= 0 && k < s.length() && s.charAt(j) == s.charAt(k)) {
            j--; k++;
        }
        if (max < k - j - 1) {
            max = k - j - 1;
            begin = j + 1;
        }
    }
}