心得：

題目要求將數字轉換為其補數，看起來滿單純的我就想不開硬幹了，由於不想借助C#內建的方法，所以自己實作二進位與十進位之間轉換。

題目：

Given a positive integer, output its complement number. The complement strategy is to flip the bits of its binary representation.

Note:

1. The given integer is guaranteed to fit within the range of a 32-bit signed integer.
2. You could assume no leading zero bit in the integer’s binary representation.

答案：

1. Normal – 硬幹

   public class Solution {
       public int FindComplement(int num) {
           string str = ConvertMethod(num);
           string ans = "";
           for(int i = 0; i < str.Length; i++){
               if(str[i] == '0'){
                   ans += "1";
               }else{
                   ans += "0";
               }
           }
           
           return ConvertMethod(ans);
       }
       
       private string ConvertMethod(int num){
           string str = "";
           while (num > 1)
           {
               str = num % 2 + str;
               num = num / 2;
               if (num == 1)
               {
                   str = num + str;
                   num = 0;
               }
           }
           
           return str;
       }
       
       private int ConvertMethod(string str)
       {
           int ans = 0;
           for (int i = 0; i < str.Length; i++)
           {
               int tmp = 1;
               for (int j = 1; j < (str.Length - i); j++)
               {
                   tmp *= 2;
               }
               ans += tmp * int.Parse(str[i].ToString());
           }
   
           return ans;
       }
   }

2. LinQ

   public class Solution {
       public int FindComplement(int num) {
           return Convert.ToInt32(new string(Convert.ToString(num, 2).Select(x => x == '1' ? '0' : '1').ToArray()), 2);
       }
   }

參考：

1. http://www.sggs.hc.edu.tw/sggsnew/comp/bccto2816.htm
2. [C#] 轉2進位 / 10進位 / 16進位