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public class Solution {
    public List<String> generatePalindromes(String s) {
        List<String> result = new ArrayList<String>();
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        for (char c : s.toCharArray()) {
            map.put(c, map.getOrDefault(c, 0) + 1);
        }
        int odd = 0;
        // The firstHalf's length is s.length() / 2
        char[] array = new char[s.length() / 2];
        int cur = 0;
        String mid = "";
        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            char key = entry.getKey();
            int freq = entry.getValue();
            if (freq % 2 == 1) {
                odd++;
                mid = mid + key;
            } 
            for (int i = 0; i < freq / 2; i++) { 
                array[cur++] = key;		
            }
        }
        if (odd > 1) {
            return result;
        }
        getPerm(result, array, mid, 0);
        return result;
    }

    private void getPerm(List<String> result, char[] array, String mid, int index) {
        if (index == array.length) {
            // String class doesn't have reverse method, so change it to StringBuffer, use reverse() and toString()
            String firstHalf = new String(array, 0, array.length);
            String secondHalf = new StringBuffer(firstHalf).reverse().toString();
            result.add(firstHalf + mid + secondHalf);
            return;
        }
        Set<Character> set = new HashSet<Character>();
        for (int i = index; i < array.length; i++) {
            if (set.add(array[i])) {
                swap(array, index, i);
                getPerm(result, array, mid, index + 1);
                swap(array, index, i);
            } 
        }
    }
    private void swap (char[] array, int i, int j) {
        char temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
}