Given four digits, find the maximum valid time that can be displayed on a digital clock (in 24-hour format) using those digits.

For example, given digits 1,8,3,2 the maximum valid time is "23:18". Note that "28:31" is not a valid time.

Write a function:

string MaxTime(int A, int B, int C, int D);

that, given four integers A,B,C,D, returns the maximum valid time in string format "HH:MM" or "NOT POSSIBLE" if it is not possible to display a valid time.

Examples: given 1,8,3,2, the function MaxTime shoud return "23:18".

Given 2,4,0,0 the funtion should return "20:40".

Given 3,0,7,0 the function should return "07:30"

Given 9,1,9,7 the function should return "NOT POSSIBLE". Since there is no possible valid time.

Assume that: A,B,C,D are integers with in the range [0..9]

In your solution, focus on correctness as well as the performance of your solution. Try to achieve O(n) if possible rather than O(n^2) solution.