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 [].

[分析]
先按照Palindrome Permutation 的方法判断输入字符串 s 能否排列成为一个回文串。通过检查开始构造过程，有两个思路：
思路1：按照leetcode的提示，我们仅需构造回文的前半部分。先利用前面判断过程中的map获得回文前半部分所有的字符，相同字符排列在一起，然后按照Permutation II一题的思路获得前半部分所有的排列情况，最后组合回文的前后部分加入结果集。
思路2：从中间往两端递归构造回文串。

[ref]
https://leetcode.com/discuss/53638/0ms-easy-c-solution
https://leetcode.com/discuss/53613/my-accepted-java-solution

public class Solution {
    // Method 1
    public List<String> generatePalindromes(String s) {
        List<String> ret = new ArrayList<String>();
        if (s == null || s.length() == 0) return ret;
        if (s.length() == 1) {ret.add(s); return ret;}
        int[] map = new int[256];
        for (int i = 0; i < s.length(); i++)
            map[s.charAt(i)]++;
        int oddCount = 0;
        int oddIdx = -1;
        StringBuilder half = new StringBuilder();
        for (int i = 0; i < 256; i++) {
            if ((map[i] & 1) == 1) {
                oddIdx = i;
                oddCount++;
                if (oddCount == 2) return ret;
            }
            int halfCount = map[i] / 2;
            for (int j = 0; j < halfCount; j++)
                half.append((char)i);
        }
        List<String> halfPermutation = new ArrayList<String>();
        getPermutation(half.toString().toCharArray(), new boolean[half.length()], new StringBuilder(), halfPermutation);
        for (String curr : halfPermutation) {
            StringBuilder curr2 = new StringBuilder(curr);
            if (oddIdx != -1)
                curr += (char)oddIdx;
            ret.add(curr + curr2.reverse().toString()); 
        }
        return ret;
    }
    
    public void getPermutation(char[] chars, boolean[] used, StringBuilder item, List<String> result) {
        if (item.length() == chars.length) {
            result.add(item.toString());
        } else {
            for (int i = 0; i < chars.length; i++) {
                if (used[i] || (i > 0 && chars[i] == chars[i - 1] && !used[i - 1]))
                    continue;
                item.append(chars[i]);
                used[i] = true;
                getPermutation(chars, used, item, result);
                used[i] = false;
                item.deleteCharAt(item.length() - 1);
            }
        }
    }
    // Method 2
    public List<String> generatePalindromes2(String s) {
        List<String> ret = new ArrayList<String>();
        int[] map = new int[256];
        for (int i = 0; i < s.length(); i++)
            map[s.charAt(i)]++;
        int oddCount = 0;
        int oddIdx = -1;
        for (int i = 0; i < 256; i++) {
            if ((map[i] & 1) == 1) {
                oddIdx = i;
                oddCount++;
                if (oddCount == 2) return ret;
            }
        }
        String curr = "";
        if (oddIdx != -1) {
            curr += (char)oddIdx;
            map[oddIdx]--;
        }
        dfs(curr, map, s.length(), ret);
        return ret;
    }
    public void dfs(String curr, int[] map, int n, List<String> ret) {
        if (curr.length() < n) {
            for (int i = 0; i < map.length; i++) {
                if (map[i] > 0) {
                    curr = (char)i + curr + (char)i;
                    map[i] -= 2;
                    dfs(curr, map, n, ret);
                    curr = curr.substring(1, curr.length() - 1);
                    map[i] += 2;
                }
            }
        } else {
            ret.add(curr);
        }
    }
}