258. Add Digits

Question

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

Example:

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

Thinking:

  • Method 1:递归
class Solution {
    public int addDigits(int num) {
        if(num < 10) return num;
        int sum = 0;
        while(num != 0){
            sum += num % 10;
            num /= 10;
        }
        return addDigits(sum);
    }
}
  • Method 2:
    • 除了0以外，从0-9循环。
class Solution {
    public int addDigits(int num) {
        return (num - 1)%9 + 1;
    }
}

Second time

  1. Recursion 
    class Solution {
 public int addDigits(int num) {
     if(num <= 9) return num;
     int sum = 0;
     while(num > 0){
         sum += num % 10;
         num /= 10;
     }
     return addDigits(sum);
 }
}