The number 1210 is unique in the sense the first digit signifies how many 0s are there, the second digit signifies how many 1s are there, the third digit signifies how many 2s are there and so on… 21200 is another such number. Can you come up with a 10-digit number? (first digit shows how many 0s are there….. all the way to tenth digit shows how many 9s are there)