low and high. A number qualifies when no digit appears twice in its decimal form: 507 qualifies, 377 does not.
Return how many qualifying numbers lie between low and high inclusive.low=95, high=105 → 8
99, 100 and 101 repeat a digit; the other 8 numbers qualify.
low=1, high=9 → 9
A one-digit number cannot repeat a digit, so all 9 qualify.
low=330, high=334 → 0
Every number from 330 to 334 has two 3s, so the answer is 0.
low ≤ high ≤ 10⁹low=95, high=105
8
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