Happy Number

Time complexity explaination:

In computer science, the number of digits in an integer is often expressed as log n, where n is the value of the integer and log is the logarithm function.

The reason for this is that the number of digits in an integer grows logarithmically with the value of the integer. Specifically, the number of digits in an integer n is roughly equal to log10(n)+1 (where log10 is the logarithm to base 10).

For example, consider the number 123456789. It has 9 digits, and log10(123456789)+1 is approximately equal to 9.09. Similarly, the number 1000000 has 7 digits, and log10(1000000)+1 is approximately equal to 7.