Finding sum of digits of a number until sum becomes single digit

To find the sum of digits of a number until it becomes a single digit, repetitively add the digits until the result is a single digit. An optimized approach involves using the mathematical formula based on modulo 9, where the result is the remainder when the number is divided by 9. If the modulo operation yields zero, the single-digit sum is 9. This method provides an efficient way to compute the repeated digital sum with constant time complexity. 

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