Medium
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:

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.

基本思路还是记得的，就是处理细节的时候还是卡住了。比如如何将写出"HH:MM"形式的时间String, 其实很简单，就String.format("%02d:%02d", hours, minutes);; 其次是如何判断String curt是不是能由String time里的characters组成。这里很巧妙用到了indexOf()方法，也是我非常不熟容易忘的方法。思维很简单，没什么好说的。

class Solution {
    public String nextClosestTime(String time) {
        int hours = Integer.parseInt(time.substring(0, 2));
        int minutes = Integer.parseInt(time.substring(3, 5));
        while (true) {
            minutes++;
            if (minutes == 60){
                minutes = 0;
                hours++;
                if (hours == 24){
                    hours = 0;
                }
            }
            String curt = String.format("%02d:%02d", hours, minutes);
            boolean valid = true;
            for (int i = 0; i < curt.length(); i++){
                if (time.indexOf(curt.charAt(i)) < 0){
                    valid = false;
                    break;
                }
            }
            if (valid){
                return curt;
            }
        }
    }
}