681. Next Closest Time

Given a time represented in the format "HH:MM", form the next closest time by reusing the current digits. There is no limit on how many times a digit can be reused.
You may assume the given input string is always valid. For example, "01:34", "12:09" are all valid. "1:34", "12:9" are all invalid.
Example 1:

Input: "19:34"
Output: "19:39"
Explanation: The next closest time choosing from 
digits 1, 9, 3, 4, is 19:39, which occurs 5 minutes later.  
It is not 19:33, because this occurs 23 hours and 59 minutes later.

Example 2:

Input: "23:59"
Output: "22:22"
Explanation: The next closest time choosing from 
digits 2, 3, 5, 9, is 22:22. 
It may be assumed that the returned time is next day's time 
since it is smaller than the input time numerically.

class Solution {
    public String nextClosestTime(String time) {
        String h = time.substring(0,2);
        String m = time.substring(3);
        int timeValue = getTimeValue(h, m);
        char[] digits = (h+m).toCharArray();
        String result = "";
        int min_diff = Integer.MAX_VALUE;
        String min_result = "";
        int min = Integer.MAX_VALUE;
        for(char a:digits){
            for(char b:digits){
                for(char c:digits){
                    for(char d:digits){
                        String newh = new String(new char[]{a,b});
                        String newm = new String(new char[]{c,d});
                        if(isValidTime(newh, newm)){
                            int diff = getTimeValue(newh, newm)-timeValue;
                            if(diff>0 && diff<min_diff){
                                min_diff = diff;
                                result = newh+":"+newm;
                            }
                            if(getTimeValue(newh, newm)<min){
                                min = getTimeValue(newh, newm);
                                min_result = newh+":"+newm;
                            }
                        }
                    }
                }
            }
        }
        if(min_diff<1500) return result;
        else return min_result;
    }
    public boolean isValidTime(String h, String m){
        int hvalue = Integer.valueOf(h);
        int mvalue = Integer.valueOf(m);
        if(hvalue>23 || hvalue<0 || mvalue<0 || mvalue>59) return false;
        return true;
    }
    public int getTimeValue(String h, String m){
        int hvalue = Integer.valueOf(h);
        int mvalue = Integer.valueOf(m);
        return hvalue*60 + mvalue;
    }
}