Java中整数数组的置换算法

Question

static ArrayList<String> permutations(String s) {
        if (s == null) {
            return null;
        }

        ArrayList<String> resultList = new ArrayList<String>();

        if (s.length() < 2) {
            resultList.add(s);

            return resultList;
        }

        int length = s.length();
        char currentChar;

        for (int i = 0; i < length; i++) {
            currentChar = s.charAt(i);

            String subString = s.substring(0, i) + s.substring(i + 1);

            ArrayList<String> subPermutations = permutations(subString);

            for (String item : subPermutations) {
                resultList.add(currentChar + item);
            }
        }

        return resultList;
    } 

static ArrayList<int[]> permutations(int[] arr) {
        ArrayList<int[]> resultList = new ArrayList<int[]>();

        if (arr.length < 2) {
            resultList.add(arr);

            return resultList;
        } 

        for (int i = 0; i < arr.length; i++) {
            int currentItem = arr[i];
            int[] newArr = new int[arr.length - 1];
            int[] newPermutation = new int[arr.length];
            int j;

//          System.arraycopy(arr, 0, newArr, 0, i);
//          System.arraycopy(arr, i + 1, newArr, i, arr.length - i - 1);

            for (j = 0; j < i; j++) {
                newArr[j] = arr[j];
            }

            for (j = i + 1; j < arr.length; j++) {
                newArr[j - 1] = arr[j];
            }

            ArrayList<int[]> subPermutations = permutations(newArr);

            newPermutation[0] = currentItem;

//          for (int i1 = 0; i1 < subPermutations.size(); i1++) {
//              for (j = 0; j < subPermutations.get(i1).length; j++) {
//                  newPermutation[j + 1] = subPermutations.get(i1)[j];
//              }
//              
//              resultList.add(newPermutation);
//          }

            for (int[] item : subPermutations) {
                for (j = 0; j < item.length; j++) {
                    newPermutation[j + 1] = item[j];
                }

                resultList.add(newPermutation);
            }

//          return resultList;
        }

        return resultList;
    }

1 4 5 
1 4 5 
5 4 1 
5 4 1 
4 5 1 
4 5 1

如果你以前遇到过这些问题，请给我一些建议。

提前感谢！

Answer 1

我在一段时间以前编写了这段代码，并对其进行了一些编辑以满足您的要求。希望能管用。

static ArrayList<String> permutations(String s) {
    ArrayList<String> ret = new ArrayList<String>();
    permutation(s.toCharArray(), 0, ret);
    return ret;
}

public static void permutation(char[] arr, int pos, ArrayList<String> list){
    if(arr.length - pos == 1)
        list.add(new String(arr));
    else
        for(int i = pos; i < arr.length; i++){
            swap(arr, pos, i);
            permutation(arr, pos+1, list);
            swap(arr, pos, i);
        }
}

public static void swap(char[] arr, int pos1, int pos2){
    char h = arr[pos1];
    arr[pos1] = arr[pos2];
    arr[pos2] = h;
}

更新
我刚在ideone.com上试过。好像管用。不用客气.：）

UPDATE 2
它应该与int数组基本相同：

static ArrayList<int[]> permutations(int[] a) {
    ArrayList<int[]> ret = new ArrayList<int[]>();
    permutation(a, 0, ret);
    return ret;
}

public static void permutation(int[] arr, int pos, ArrayList<int[]> list){
    if(arr.length - pos == 1)
        list.add(arr.clone());
    else
        for(int i = pos; i < arr.length; i++){
            swap(arr, pos, i);
            permutation(arr, pos+1, list);
            swap(arr, pos, i);
        }
}

public static void swap(int[] arr, int pos1, int pos2){
    int h = arr[pos1];
    arr[pos1] = arr[pos2];
    arr[pos2] = h;
}