Count Happy Numbers Below N

Problem

Count how many happy numbers strictly less than n exist, where a happy number reaches 1 by repeatedly replacing it with the sum of the squares of its digits.

Run the digit-square process with a seen set for every value from 1 to n - 1. Return 0 for n <= 1.

Hint

1 is happy, and unhappy numbers fall into a cycle that the seen set detects.

Test cases

case 1

in: [1]

expected: 0

case 2

in: [2]

expected: 1

case 3

in: [10]

expected: 2

case 4

in: [20]

expected: 5

case 5

in: [100]

expected: 19

Starter codePython

def count_happy_below(n):
    # Your code here
    pass

5 test cases · run your code against them in the editor