这里有一个优雅的，非递归的O(n!)解:

public static StringBuilder[] permutations(String s) {
        if (s.length() == 0)
            return null;
        int length = fact(s.length());
        StringBuilder[] sb = new StringBuilder[length];
        for (int i = 0; i < length; i++) {
            sb[i] = new StringBuilder();
        }
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            int times = length / (i + 1);
            for (int j = 0; j < times; j++) {
                for (int k = 0; k < length / times; k++) {
                    sb[j * length / times + k].insert(k, ch);
                }
            }
        }
        return sb;
    }

为排列和组合添加更详细的NcK/NcR

public static void combinationNcK(List<String> inputList, String prefix, int chooseCount, List<String> resultList) {
    if (chooseCount == 0)
        resultList.add(prefix);
    else {
        for (int i = 0; i < inputList.size(); i++)
            combinationNcK(inputList.subList(i + 1, inputList.size()), prefix + "," + inputList.get(i), chooseCount - 1, resultList);

        // Finally print once all combinations are done
        if (prefix.equalsIgnoreCase("")) {
            resultList.stream().map(str -> str.substring(1)).forEach(System.out::println);
        }
    }
}

public static void permNcK(List<String> inputList, int chooseCount, List<String> resultList) {
    for (int count = 0; count < inputList.size(); count++) {
        permNcK(inputList, "", chooseCount, resultList);
        resultList = new ArrayList<String>();
        Collections.rotate(inputList, 1);
        System.out.println("-------------------------");
    }

}

public static void permNcK(List<String> inputList, String prefix, int chooseCount, List<String> resultList) {
    if (chooseCount == 0)
        resultList.add(prefix);
    else {
        for (int i = 0; i < inputList.size(); i++)
            combinationNcK(inputList.subList(i + 1, inputList.size()), prefix + "," + inputList.get(i), chooseCount - 1, resultList);

        // Finally print once all combinations are done
        if (prefix.equalsIgnoreCase("")) {
            resultList.stream().map(str -> str.substring(1)).forEach(System.out::println);
        }
    }
}

public static void main(String[] args) {
    List<String> positions = Arrays.asList(new String[] { "1", "2", "3", "4", "5", "6", "7", "8" });
    List<String> resultList = new ArrayList<String>();
    //combinationNcK(positions, "", 3, resultList);

    permNcK(positions, 3, resultList);

}

//Rotate and create words beginning with all letter possible and push to stack 1

//Read from stack1 and for each word create words with other letters at the next location by rotation and so on 

/*  eg : man

    1. push1 - man, anm, nma
    2. pop1 - nma ,  push2 - nam,nma
       pop1 - anm ,  push2 - amn,anm
       pop1 - man ,  push2 - mna,man
*/

public class StringPermute {

    static String str;
    static String word;
    static int top1 = -1;
    static int top2 = -1;
    static String[] stringArray1;
    static String[] stringArray2;
    static int strlength = 0;

    public static void main(String[] args) throws IOException {
        System.out.println("Enter String : ");
        InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader bfr = new BufferedReader(isr);
        str = bfr.readLine();
        word = str;
        strlength = str.length();
        int n = 1;
        for (int i = 1; i <= strlength; i++) {
            n = n * i;
        }
        stringArray1 = new String[n];
        stringArray2 = new String[n];
        push(word, 1);
        doPermute();
        display();
    }

    public static void push(String word, int x) {
        if (x == 1)
            stringArray1[++top1] = word;
        else
            stringArray2[++top2] = word;
    }

    public static String pop(int x) {
        if (x == 1)
            return stringArray1[top1--];
        else
            return stringArray2[top2--];
    }

    public static void doPermute() {

        for (int j = strlength; j >= 2; j--)
            popper(j);

    }

    public static void popper(int length) {
        // pop from stack1 , rotate each word n times and push to stack 2
        if (top1 > -1) {
            while (top1 > -1) {
                word = pop(1);
                for (int j = 0; j < length; j++) {
                    rotate(length);
                    push(word, 2);
                }
            }
        }
        // pop from stack2 , rotate each word n times w.r.t position and push to
        // stack 1
        else {
            while (top2 > -1) {
                word = pop(2);
                for (int j = 0; j < length; j++) {
                    rotate(length);
                    push(word, 1);
                }
            }
        }

    }

    public static void rotate(int position) {
        char[] charstring = new char[100];
        for (int j = 0; j < word.length(); j++)
            charstring[j] = word.charAt(j);

        int startpos = strlength - position;
        char temp = charstring[startpos];
        for (int i = startpos; i < strlength - 1; i++) {
            charstring[i] = charstring[i + 1];
        }
        charstring[strlength - 1] = temp;
        word = new String(charstring).trim();
    }

    public static void display() {
        int top;
        if (top1 > -1) {
            while (top1 > -1)
                System.out.println(stringArray1[top1--]);
        } else {
            while (top2 > -1)
                System.out.println(stringArray2[top2--]);
        }
    }
}

没有递归的Java实现

public Set<String> permutate(String s){
    Queue<String> permutations = new LinkedList<String>();
    Set<String> v = new HashSet<String>();
    permutations.add(s);

    while(permutations.size()!=0){
        String str = permutations.poll();
        if(!v.contains(str)){
            v.add(str);
            for(int i = 0;i<str.length();i++){
                String c = String.valueOf(str.charAt(i));
                permutations.add(str.substring(i+1) + c +  str.substring(0,i));
            }
        }
    }
    return v;
}

其中一个简单的解决方案是使用两个指针继续递归地交换字符。

public static void main(String[] args)
{
    String str="abcdefgh";
    perm(str);
}
public static void perm(String str)
{  char[] char_arr=str.toCharArray();
    helper(char_arr,0);
}
public static void helper(char[] char_arr, int i)
{
    if(i==char_arr.length-1)
    {
        // print the shuffled string 
            String str="";
            for(int j=0; j<char_arr.length; j++)
            {
                str=str+char_arr[j];
            }
            System.out.println(str);
    }
    else
    {
    for(int j=i; j<char_arr.length; j++)
    {
        char tmp = char_arr[i];
        char_arr[i] = char_arr[j];
        char_arr[j] = tmp;
        helper(char_arr,i+1);
        char tmp1 = char_arr[i];
        char_arr[i] = char_arr[j];
        char_arr[j] = tmp1;
    }
}
}

递归Python解决方案

def permute(input_str):
    _permute("", input_str)

def _permute(prefix, str_to_permute):
    if str_to_permute == '':
        print(prefix)

    else:
        for i in range(len(str_to_permute)): 
            _permute(prefix+str_to_permute[i], str_to_permute[0:i] + str_to_permute[i+1:])

if __name__ == '__main__':
    permute('foobar')