[Swift]LeetCode878. 第 N 個神奇數字 | Nth Magical Number

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A positive integer is magical if it is divisible by either A or B.

Return the N-th magical number.  Since the answer may be very large, return it modulo 10^9 + 7. 

Example 1:

Input: N = 1, A = 2, B = 3 Output: 2 

Example 2:

Input: N = 4, A = 2, B = 3 Output: 6 

Example 3:

Input: N = 5, A = 2, B = 4 Output: 10 

Example 4:

Input: N = 3, A = 6, B = 4 Output: 8 

Note:

1. 1 <= N <= 10^9
2. 2 <= A <= 40000
3. 2 <= B <= 40000

---

若是正整數能夠被 A 或 B 整除，那麼它是神奇的。

返回第 N 個神奇數字。因爲答案可能很是大，返回它模 10^9 + 7 的結果。 

示例 1：

輸入：N = 1, A = 2, B = 3
輸出：2

示例 2：

輸入：N = 4, A = 2, B = 3
輸出：6

示例 3：

輸入：N = 5, A = 2, B = 4
輸出：10

示例 4：

輸入：N = 3, A = 6, B = 4
輸出：8 

提示：

1. 1 <= N <= 10^9
2. 2 <= A <= 40000
3. 2 <= B <= 40000

---

Runtime: 4 ms

Memory Usage: 18.4 MB

 1 class Solution {
 2     func nthMagicalNumber(_ N: Int, _ A: Int, _ B: Int) -> Int {
 3         var a:Int = A
 4         var b:Int = B
 5         var tmp:Int = 0
 6         var l:Int = 2
 7         var r:Int = Int(1e14)
 8         var mod:Int = Int(1e9 + 7)
 9         while (b > 0)
10          {
11             tmp = a
12             a = b
13             b = tmp % b
14         }
15         while (l < r)
16         {
17             var m:Int = (l + r) / 2;
18             if m / A + m / B - m / (A * B / a) < N
19             {
20                 l = m + 1
21             }
22             else
23             {
24                 r = m
25             }
26         }
27         return Int(l % mod)
28     }
29 }