使用递归。

依次尝试每个字母作为第一个字母，然后使用递归调用找到剩余字母的所有排列。 基本情况是，当输入是空字符串时，唯一的排列就是空字符串。

Java中一个非常基本的解决方案是使用递归+设置(以避免重复)，如果你想存储和返回解决方案字符串:

public static Set<String> generatePerm(String input)
{
    Set<String> set = new HashSet<String>();
    if (input == "")
        return set;

    Character a = input.charAt(0);

    if (input.length() > 1)
    {
        input = input.substring(1);

        Set<String> permSet = generatePerm(input);

        for (String x : permSet)
        {
            for (int i = 0; i <= x.length(); i++)
            {
                set.add(x.substring(0, i) + a + x.substring(i));
            }
        }
    }
    else
    {
        set.add(a + "");
    }
    return set;
}

这是另一个更简单的方法来做一个字符串的排列。

public class Solution4 {
public static void main(String[] args) {
    String  a = "Protijayi";
  per(a, 0);

}

static void per(String a  , int start ) {
      //bse case;
    if(a.length() == start) {System.out.println(a);}
    char[] ca = a.toCharArray();
    //swap 
    for (int i = start; i < ca.length; i++) {
        char t = ca[i];
        ca[i] = ca[start];
        ca[start] = t;
        per(new String(ca),start+1);
    }

}//per

}

我的实现基于Mark Byers上面的描述:

    static Set<String> permutations(String str){
        if (str.isEmpty()){
            return Collections.singleton(str);
        }else{
            Set <String> set = new HashSet<>();
            for (int i=0; i<str.length(); i++)
                for (String s : permutations(str.substring(0, i) + str.substring(i+1)))
                    set.add(str.charAt(i) + s);
            return set;
        }
    }

这个没有递归

public static void permute(String s) {
    if(null==s || s.isEmpty()) {
        return;
    }

    // List containing words formed in each iteration 
    List<String> strings = new LinkedList<String>();
    strings.add(String.valueOf(s.charAt(0))); // add the first element to the list

     // Temp list that holds the set of strings for 
     //  appending the current character to all position in each word in the original list
    List<String> tempList = new LinkedList<String>(); 

    for(int i=1; i< s.length(); i++) {

        for(int j=0; j<strings.size(); j++) {
            tempList.addAll(merge(s.charAt(i), strings.get(j)));
                        }
        strings.removeAll(strings);
        strings.addAll(tempList);

        tempList.removeAll(tempList);

    }

    for(int i=0; i<strings.size(); i++) {
        System.out.println(strings.get(i));
    }
}

/**
 * helper method that appends the given character at each position in the given string 
 * and returns a set of such modified strings 
 * - set removes duplicates if any(in case a character is repeated)
 */
private static Set<String> merge(Character c,  String s) {
    if(s==null || s.isEmpty()) {
        return null;
    }

    int len = s.length();
    StringBuilder sb = new StringBuilder();
    Set<String> list = new HashSet<String>();

    for(int i=0; i<= len; i++) {
        sb = new StringBuilder();
        sb.append(s.substring(0, i) + c + s.substring(i, len));
        list.add(sb.toString());
    }

    return list;
}

下面是一个java实现:

/* All Permutations of a String */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Complexity O(n*n!) */
class Ideone
{
     public static ArrayList<String> strPerm(String str, ArrayList<String> list)
     {
        int len = str.length();
        if(len==1){
            list.add(str);
            return list;
        }

        list = strPerm(str.substring(0,len-1),list);
        int ls = list.size();
        char ap = str.charAt(len-1);
        for(int i=0;i<ls;i++){
            String temp = list.get(i);
            int tl = temp.length();
            for(int j=0;j<=tl;j++){
                list.add(temp.substring(0,j)+ap+temp.substring(j,tl));  
            }
        }

        while(true){
            String temp = list.get(0);
            if(temp.length()<len)
                list.remove(temp);
            else
                break;
        }

        return list;
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        String str = "abc";
        ArrayList<String> list = new ArrayList<>();

        list = strPerm(str,list);
        System.out.println("Total Permutations : "+list.size());
        for(int i=0;i<list.size();i++)
            System.out.println(list.get(i));

    }
}

http://ideone.com/nWPb3k