字符串的排列

import java.util.*;
public class Solution {
    public ArrayList<String> Permutation(String str) {
        
        ArrayList<String> ret = new ArrayList<String>();
        if(str == null || str.length() <= 0)
            return ret;
        dfs(str.toCharArray(), 0, ret);
        Collections.sort(ret);
        
        return ret;
       
    }
    
    public void dfs(char[] chs, int i, ArrayList<String> ret) {
        
        if(i == chs.length - 1) {
            ret.add(String.valueOf(chs));
        }
        else {
            for(int j=i; j<chs.length; j++) {
                if(i == j || chs[i] != chs[j]) {
                    swap(chs, i, j);
                    dfs(chs, i+1, ret);
                    swap(chs, i, j);
                }
            }
        }
        
    }
    
    public void swap(char[] chs, int i, int j) {
        char ch = chs[i];
        chs[i] = chs[j];
        chs[j] = ch;
    }
}