leetcode@ [336] Palindrome Pairs (HashMap)

https://leetcode.com/problems/palindrome-pairs/

Given a list of unique words. Find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e. words[i] + words[j] is a palindrome.

Example 1:
Given words = ["bat", "tab", "cat"]
Return [[0, 1], [1, 0]]
The palindromes are ["battab", "tabbat"]

Example 2:
Given words = ["abcd", "dcba", "lls", "s", "sssll"]
Return [[0, 1], [1, 0], [3, 2], [2, 4]]
The palindromes are ["dcbaabcd", "abcddcba", "slls", "llssssll"]

public class Solution {

    public static boolean isPalindrome(StringBuffer sb) {

        if(sb == null || sb.length() == 0)  return true;

        int l = 0, r = sb.length()-1;

        while(l < r) {
            if(sb.charAt(l) != sb.charAt(r))  return false;
            ++l;  --r;
        }

        return true;
    }

    public static void buildMappings(String[] words, HashMap<String, HashSet<String> >  mapping) {

        for(int i=0; i<words.length; ++i) {

            StringBuffer sb = new StringBuffer(words[i]);
            HashSet<String> adj = new HashSet<String> ();

            for(int j=0; j<=sb.length(); ++j) {
                /** partition words[i] into two parts */
                StringBuffer prefix = new StringBuffer(sb.substring(0, j));
                StringBuffer suffix = new StringBuffer(sb.substring(j));

                /** check whether or not the prefix of words[i] is palindrome */
                /** if it does, then the prefix can be the rotated point, otherwise not*/
                if(isPalindrome(prefix)) {
                    /** adj stores the strings where suffix + words[i] is palindrome */
                    adj.add(suffix.reverse().toString());
                } 

                if(isPalindrome(suffix)) {
                    /** adj stores the strings where words[i] + prefix is palindrome */
                    adj.add(prefix.reverse().toString());
                }
            }
            /** add it to mapping */
            mapping.put(sb.toString(), adj);

        }

        HashSet<String> hs = new HashSet<String> ();
        for(int i=0; i<words.length; ++i) {

            StringBuffer sb = new StringBuffer(words[i]);

            if(isPalindrome(sb)) {
                hs.add(sb.toString());
            }
        }

        for(int i=0; i<words.length; ++i) {

            if(words[i].equals("")) {
                HashSet<String> adj = new HashSet<String> ();
                adj.addAll(hs);
                mapping.put(words[i], adj);
            }
        }

    }

    public List<List<Integer>> palindromePairs(String[] words) {

        HashSet<List<Integer> > hres = new HashSet<List<Integer> > ();
        List<List<Integer> > res = new ArrayList<List<Integer> > ();

        HashMap<String, HashSet<String> >  mapping = new HashMap<String, HashSet<String> > ();
        buildMappings(words, mapping);

        HashMap<String, Integer> dict = new HashMap<String, Integer> ();
        for(int i=0; i<words.length; ++i) {
            dict.put(words[i], i);
        }

        Iterator iter = mapping.entrySet().iterator();
        while(iter.hasNext()) {
            HashMap.Entry entry = (HashMap.Entry) iter.next();

            String str = (String) entry.getKey();
            int lpos = dict.get(str);
            //System.out.print(str + " ==> ");
            HashSet<String> hs = (HashSet<String>) entry.getValue();
            Iterator<String> itc = hs.iterator();

            while(itc.hasNext()) {
                String s = itc.next();

                //System.out.print(s + "  ");

                if(dict.containsKey(s)) {
                    int rpos = dict.get(s);
                    if(lpos != rpos) {

                        StringBuffer sb1 = new StringBuffer(str).append(s);
                        if(isPalindrome(sb1)) {
                            List<Integer> ll = new ArrayList<Integer> ();
                            ll.add(lpos);
                            ll.add(rpos);
                            hres.add(ll);
                        }

                        StringBuffer sb2 = new StringBuffer(s).append(str);
                        if(isPalindrome(sb2)) {
                            List<Integer> ll2 = new ArrayList<Integer> ();
                            ll2.add(rpos);
                            ll2.add(lpos);
                            hres.add(ll2);
                        }
                    }
                }

            }//System.out.println();
        }

        res.addAll(hres);

        return res;
    }
}