给定字符串str，搜索第m个只出现n次的字符

给定字符串"tervest"，搜索第2个只出现1次的字符，输出v。

public static String searchMNChar(String str,int m,int n){

  int mCount =0;

  HashSet singleChar =new HashSet<>(str.length());

  for (int i =0 ; i < str.length() ; i ++){

    if (singleChar.contains(str.charAt(i))){

      continue;

    }

    singleChar.add(str.charAt(i));

    int nCount =0;

    for (int j = i; j < str.length(); j++) {

       if (str.charAt(i) == str.charAt(j)){

          nCount++;

      }

    }

    if (nCount == n){

      mCount++;

    }

    if (mCount == m){

      return str.substring(i,++i);

    }

  }

  return null;

}