字符串的全排列

递归，注意还原，注意从当前字符开始，即i=from
public static void permutation(char[] ch, int from, int to){
    if(to <= from){
        System.out.println(String.valueOf(ch));
    } else{
        for(int i = from; i <= to; i++){
            char temp = ch[from];
            ch[from] = ch[i];
            ch[i] = temp;
            permutation(ch,from+1,to);
            temp = ch[from];
            ch[from] = ch[i];
            ch[i] = temp;
        }
    }
}