字符串组合

package test;
import java.util.LinkedList;

public class TestCombination {
    public static void combination(char[] arr) {
        int len = arr.length;
        int[] used = new int[len];
        LinkedList<Character> linkedlist = new LinkedList<Character>();
        combination(arr, 0, linkedlist, used);
    }

    public static void combination(char[] arr, int i,
            LinkedList<Character> linkedlist, int[] used) {
        if (i == arr.length) {
            for (int j = 0; j < linkedlist.size(); j++) {
                System.out.print(linkedlist.get(j) + " ");
            }
            System.out.println();
        } else {
            for (int k = 0; k < arr.length; k++) {
                if (used[k] == 0) {
                    used[k] = 1;
                    linkedlist.push(arr[k]);
                    combination(arr, i + 1, linkedlist, used);
                    linkedlist.pop();
                    used[k] = 0;
                }
            }
        }
    }
    public static void combination_norepeat(String str){
        char[] charr=str.toCharArray();
        int [] used=new int[255];
        int diffcharnum=0;
        for(int i=0;i<charr.length ;i++){
            used[charr[i]]++;
        }
        char[] res=new char[charr.length];
        for(int i=0;i<255;i++){
            if(used[i]>0){
                diffcharnum++;
            }
        }
        char[] letters=new char[diffcharnum];  //统计不同的字符
        for(int i=0,j=0;i<255&&j<diffcharnum;i++){
            if(used[i]>0){
                letters[j++]=(char) i;
            }
        }
        combination_norepeat(charr,0,used,res,letters);
    }
    public static void combination_norepeat(char [] str,int i,int [] used,char [] res,char[] letters){
        if(i==res.length ){
            for(int j=0;j<res.length;j++){
                System.out.print(res[j]+" ");
            }
            System.out.println();
        }else{
            for(int k=0;k<letters.length;k++){
                if(used[letters[k]]>0){
                    used[letters[k]]--;
                    res[i]=letters[k];
                    combination_norepeat(str,i+1,used,res,letters);
                    used[letters[k]]++;
                }
            }
        }
    }
    public static int AddWithoutArithmetic(int num1, int num2) {
        if (num2 == 0)
            return num1;
        int sum = num1 ^ num2;
        int carry = (num1 & num2) << 1;
        return AddWithoutArithmetic(sum, carry);
    }

    public static void main(String args[]) {
        String str = "aabc";
        combination_norepeat(str);
    }
}