878. Nth Magical Number 
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Problem
A positive integer is magical if it is divisible by either a or b.
Given the three integers n, a, and b, return the nth magical number. Since the answer may be very large, return it modulo 109 + 7.
Example 1:
Input: n = 1, a = 2, b = 3 Output: 2
Example 2:
Input: n = 4, a = 2, b = 3 Output: 6
Constraints:
1 <= n <= 1092 <= a, b <= 4 * 104