LeetCode-Palindrome Permutation II

Given a string s, return all the palindromic permutations (without duplicates) of it. Return an empty list if no palindromic permutation could be form.

For example:

Given s = "aabb", return ["abba", "baab"].

Given s = "abc", return [].

Analysis:

Should ask if the chars contains only letters, lowercase letters or any char.

Solution:

import java.util.Map.Entry;

public class Solution {
    public List<String> generatePalindromes(String s) {
        List<String> resList = new ArrayList<String>();
        if (s.isEmpty())
            return resList;

        HashMap<Character, Integer> charMap = new HashMap<Character, Integer>();
        for (int i = 0; i < s.length(); i++) {
            char curChar = s.charAt(i);
            int count = charMap.getOrDefault(curChar, 0);
            charMap.put(curChar, count + 1);
        }
        int[] counts = new int[charMap.size()];
        char[] chars = new char[charMap.size()];
        int ind = 0;
        for (Entry entry : charMap.entrySet()) {
            chars[ind] = (char) entry.getKey();
            counts[ind++] = (int) entry.getValue();
        }

        // If there is more than one odd count, fail.
        boolean hasOdd = false;
        int oddIndex = -1;
        for (int i = 0; i < counts.length; i++)
            if (counts[i] % 2 == 1) {
                if (hasOdd)
                    return resList;
                hasOdd = true;
                oddIndex = i;
            }

        StringBuilder builder = new StringBuilder();

        int leftLen = s.length();
        if (hasOdd) {
            char c = chars[oddIndex];
            counts[oddIndex]--;
            builder.append(c);
            leftLen--;
        }

        getPalindromes(chars, counts, leftLen, builder, resList);
        return resList;
    }

    public void getPalindromes(char[] chars, int[] counts, int leftLen, StringBuilder builder,
            List<String> resList) {
        if (leftLen == 0) {
            resList.add(builder.toString());
            return;
        }

        for (int i = 0; i < counts.length; i++)
            if (counts[i] > 0) {
                char curChar = chars[i];
                counts[i] -= 2;
                builder.insert(0, curChar);
                builder.append(curChar);
                getPalindromes(chars, counts, leftLen - 2, builder, resList);
                builder.deleteCharAt(builder.length() - 1);
                builder.deleteCharAt(0);
                counts[i] += 2;
            }
    }
}