[C#][LeetCode][Easy] 258. Add Digits

心得：

題目要求將每一位數相加，直到剩下個位數。

問題：

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

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

答案：

1. For + 遞迴
   
   C#

   public class Solution { public int AddDigits(int num) { string str = num.ToString(); if (str.Length < 2) { return int.Parse(str); } else { int tmp = 0; for (int i = 0; i < str.Length; i++) { tmp += int.Parse(str[i].ToString()); } return AddDigits(tmp); } } }

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   public class Solution {     public int AddDigits(int num) {         string str = num.ToString();         if (str.Length < 2)         {             return int.Parse(str);         }         else         {             int tmp = 0;             for (int i = 0; i < str.Length; i++)             {                 tmp += int.Parse(str[i].ToString());             }                  return AddDigits(tmp);         }     } }

2. LinQ + 遞迴
   
   C#

   public class Solution { public int AddDigits(int num) { string str = num.ToString(); if (str.Length < 2) { return int.Parse(str); } else { return AddDigits(str.Sum(x => int.Parse(x.ToString()))); } } }

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   public class Solution {     public int AddDigits(int num) {         string str = num.ToString();         if (str.Length < 2)         {             return int.Parse(str);         }         else         {             return AddDigits(str.Sum(x => int.Parse(x.ToString())));         }     } }

3. Normal
   
   C#

   public class Solution { public int AddDigits(int num) { string str = num.ToString(); while (str.Length > 1) { int tmp = 0; for (int i = 0; i < str.Length; i++) { tmp += int.Parse(str[i].ToString()); } str = tmp.ToString(); } return int.Parse(str); } }

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   public class Solution {     public int AddDigits(int num) {         string str = num.ToString();         while (str.Length > 1)         {             int tmp = 0;             for (int i = 0; i < str.Length; i++)             {                 tmp += int.Parse(str[i].ToString());             }               str = tmp.ToString();         }           return int.Parse(str);     } }