递归求字符串中出现最多的字母

求一个z字符串中出现次数最多的那个字母及次数，如有多个重复则一起求出

public class ArrayChar {    @Test    public void demo1(){        String input ="aavzadfdsgfsdhshgwasdfasdfdddaaa";        doString(input);    }    public void doString(String input){        char[] chars = input.toCharArray();      /*  for (char c:chars) {            System.out.println(c);        }*/        ArrayList list = new ArrayList();        TreeSet set = new TreeSet();//去除重复的字母        for(int i =0;i<chars.length;i++){            list.add(String.valueOf(chars[i]));            set.add(String.valueOf(chars[i]));        }        System.out.println(set);        Collections.sort(list);        System.out.println(list);        Iterator iterator = set.iterator();        int max = 0;        int first,last,length;        String maxChar = null;        while(iterator.hasNext()){           String key = (String) iterator.next();            first = list.indexOf(key);            last = list.lastIndexOf(key);            length = last - first + 1;            if(length>max){                max = length;                maxChar = key;            }            else if (length == max){                maxChar = maxChar+" "+key;            }        }        System.out.println(max+" "+maxChar);    }}
输出：
[a, d, f, g, h, s, v, w, z][a, a, a, a, a, a, a, a, d, d, d, d, d, d, d, d, f, f, f, f, g, g, h, h, s, s, s, s, s, v, w, z]8 a d