Teaching Kids Programming: Videos on Data Structures and Algorithms
Write an algorithm to determine if a number n is happy. A happy number is a number defined by the following process:
- Starting with any positive integer, replace the number by the sum of the squares of its digits.
- Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
- Those numbers for which this process ends in 1 are happy.
- Return true if n is a happy number, and false if not.
Example 1:
Input: n = 19
Output: true
Explanation:
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1Example 2:
Input: n = 2
Output: false
Check Happy Number Algorithms
Using a set to keep tracking the numbers we have generated. Then, for each number, we extract the digits by digits, square it, and compute the sum until it either is 1 or it has appeared in the set.
1 2 3 4 5 6 7 8 9 10 11 12 | class Solution: def isHappy(self, n: int) -> bool: d = set() while n != 1 and (not n in d): d.add(n) cur = n a = 0 while cur > 0: a += (cur % 10)**2 cur //= 10 n = a return n == 1 |
class Solution: def isHappy(self, n: int) -> bool: d = set() while n != 1 and (not n in d): d.add(n) cur = n a = 0 while cur > 0: a += (cur % 10)**2 cur //= 10 n = a return n == 1
We can convert the number into the string, then iterate over the digits, convert it back to integer, square it…
1 2 3 4 5 6 7 8 9 10 | class Solution: def isHappy(self, n: int) -> bool: d = set() while n != 1 and (not n in d): d.add(n) a = 0 for i in str(n): a += int(i)**2 n = a return n == 1 |
class Solution: def isHappy(self, n: int) -> bool: d = set() while n != 1 and (not n in d): d.add(n) a = 0 for i in str(n): a += int(i)**2 n = a return n == 1
We can use the list comprehension to make the code shorter.
1 2 3 4 5 6 7 | class Solution: def isHappy(self, n: int) -> bool: d = set() while n != 1 and (not n in d): d.add(n) n = sum([int(x)**2 for x in str(n)]) return n == 1 |
class Solution: def isHappy(self, n: int) -> bool: d = set() while n != 1 and (not n in d): d.add(n) n = sum([int(x)**2 for x in str(n)]) return n == 1
See also: Happy Number Detection Algorithm using Hash Set
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