2 lines Python with explanation

  • 0
    class Solution(object):
        def addDigits(self, num):
            if num==0: return 0
            return 1 + (num-1)%9

    Explanation (from Wolfram Alpha):

    The digital root dr(n) of an integer n can be computed without actually performing the iteration using the simple congruence formula:

    Congruence formula

    = 1 + [n-1(mod 9)]

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