258 - Add Digits

258. Add Digits

Problem Statement

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

 

Example 1:

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2
Since 2 has only one digit, return it.

Example 2:

Input: num = 0
Output: 0

 

Constraints:

• 0 <= num <= 231 - 1

 

Follow up: Could you do it without any loop/recursion in O(1) runtime?

Click to open Hints

• A naive implementation of the above process is trivial. Could you come up with other methods?
• What are all the possible results?
• How do they occur, periodically or randomly?
• You may find this Wikipedia article useful.

Solutions

Rust

impl Solution {
    pub fn add_digits(num: i32) -> i32 {
        let mut num = num;

        // loop while the number is greater than 9
        while num > 9 {
            let mut sum = 0;
            // loop while the number is greater than 0,
            // add the last digit to the sum,
            // and divide the number by 10
            while num > 0 {
                sum += num % 10;
                num /= 10;
            }
            // set the number to the sum
            num = sum;
        }

        num
    }
}

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