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102345. We can't start with 0 because that would make it a 5 digit number. Start with "1" because it is the next smallest, then use the 0 because it will reduce the number the most when in the tens of thousands digit. Then use the smallest from there on, giving us 2,3,4,5.
1,111,111,111
-9876543 or, if only positive number are permitted, 1023456.
The largest digits number with only 2 digits alike is 99
9876543210
987654321 is the highest nine-digit number with no repeated digits.