LeetCode Entry
258. Add Digits
It is just an array pointer loop shifted by 1.
258. Add Digits easy
fun addDigits(num: Int): Int = if (num == 0) 0 else 1 + ((num - 1) % 9)
// 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
// 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2
// 0 [1..9] [1..9] [1..9] ...
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Intuition
Observing the pattern:
// 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
// 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2
// 0 [1..9] [1..9] [1..9] ...
There is a repeating part of it: [1..9], so we can derive the formula.
Approach
It is just an array pointer loop shifted by 1.
Complexity
- Time complexity: \(O(1)\)
- Space complexity: \(O(1)\)