258. Add Digits.md

258. Add Digits

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leetcode Daily Challenge on July 26th, 2020.

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Difficulty : Easy

Related Topics : Math

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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.

Follow up:

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

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Solution

• mine
  • Java
    • Runtime: 1 ms, faster than 100.00%, Memory Usage: 36.2 MB, less than 97.48% of Java online submissions

      //O(N^2)time   N is num.length
      //O(1)space
      public int addDigits(int num) {
          int res = num;
          while(res > 9){
              int t = 0;
              while(res > 0){
                  t += res % 10;
                  res /= 10;
              }
              res = t;
          }
          return res;
      }
      

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