class Solution {
    
    public String largestTimeFromDigits(int[] A) {
        List<Integer> digits =Arrays.stream(A).boxed().collect(Collectors.toList());
        List<Integer> permutations = new ArrayList<Integer>();
        permutations = permute(digits,0,permutations);
        int result = -1;
        for (Integer i: permutations){
            if(i % 100 < 60 && i / 100 < 24 && i > result){
                result = i;
            }
        }
        return result == -1? "" : "" + String.format("%02d",result / 100) + ":" + String.format("%02d",result % 100);
    }

    List<Integer> permute(List<Integer> arr, int k, List<Integer> result){
        for(int i = k; i < arr.size(); i++){
            java.util.Collections.swap(arr, i, k);
            permute(arr, k+1, result);
            java.util.Collections.swap(arr, k, i);
        }
        if (k == arr.size() -1){
            result.add(Integer.parseInt(""+arr.get(0) + arr.get(1) + arr.get(2) + arr.get(3)));
        }
        return result;
    }
}