2748 Number of Beautiful Pairs

考点比较数学.

Approach 1

复习了三个知识点:

• math.gcd(a,b) 求最大公约数 (greatest common divisor), if gcd(a,b) == 1 说明a,b互质
• last digit of a number num % 10
• first digit of a number num // 10 until num < 10

import math
class Solution:
    def countBeautifulPairs(self, nums: List[int]) -> int:
        """
        1. enumeration
        2. math.gcd()
        """
        pairs = 0
        for j, num in enumerate(nums):
            # last digit for nums[j]
            last_digit = num % 10
            for i in range(j):
                first_digit = nums[i]
                # get first digit
                while first_digit >= 10:
                    first_digit //= 10
                pairs += math.gcd(last_digit,first_digit) == 1
        return pairs