1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 namespace  bigIntMultiply {      class  Program      {          static  void  Main( string [] args)          {             string  a =  "99999999999999" ;             string  b =  "123456789001234567890" ;              Stopwatch sw =  new  Stopwatch();              sw.Start();              string  s = Multiply(b, b);              sw.Stop();              Console.WriteLine(s);              Console.WriteLine(sw.Elapsed);          }          //字符串模拟乘法操作          static  string  Multiply( string  x,  string  y)          {              //deep++;// Console.WriteLine("-" + deep + "-");              string  negative =  "" ;              if  ((x.StartsWith( "-" ) && y.StartsWith( "-" )) || (!x.StartsWith( "-" ) && !y.StartsWith( "-" )))              {                  x = x.TrimStart( '-' ); y = y.TrimStart( '-' );                  negative =  "" ;              }              else  if  ((x.StartsWith( "-" ) && !y.StartsWith( "-" )) || (!x.StartsWith( "-" ) && y.StartsWith( "-" )))              {                  x = x.TrimStart( '-' ); y = y.TrimStart( '-' );                  negative =  "-" ;              }              //如果长度都小于9，直接相乘，返回就行了。              if  (x.Length <= 9 && y.Length <= 9)              {                  long  tmp = ( long .Parse(x) *  long .Parse(y));                  if  (tmp == 0)                      return  tmp.ToString();                  return  negative + ( long .Parse(x) *  long .Parse(y)).ToString();              }              //公式里的abcd              string  a, b, c, d;              if  (x.Length <= 9)              {                  a =  "0" ; b = x;              }              else              {                  if  (x.Length % 2 != 0)                      x =  "0"  + x;                  a = x.Substring(0, x.Length / 2);                  b = x.Substring(x.Length / 2);              }              if  (y.Length <= 9)              {                  c =  "0" ;                  d = y;              }              else              {                  if  (y.Length % 2 != 0)                      y =  "0"  + y;                  c = y.Substring(0, y.Length / 2);                  d = y.Substring(y.Length / 2);              }              int  n = x.Length >= y.Length ? x.Length : y.Length;              string  t1, t2, t3;              //递归调用，根据公式计算出值。              string  ac = Multiply(a, c);              string  bd = Multiply(b, d);              t1 = Multiply(Subtract(a, b), Subtract(d, c));              t2 = Add(Add(t1, ac), bd);              t3 = Add(Add(Power10(ac, n), Power10(t2, n / 2)), bd).TrimStart( '0' );              if  (t3 ==  "" )  return  "0" ;              return  negative + t3;          }          //字符串模拟加法操作          static  string  Add( string  x,  string  y)          {              if  (x.StartsWith( "-" ) && !y.StartsWith( "-" ))              {                  return  Subtract(y, x.TrimStart( '-' ));              }              else  if  (!x.StartsWith( "-" ) && y.StartsWith( "-" ))              {                  return  Subtract(x, y.TrimStart( '-' ));              }              else  if  (x.StartsWith( "-" ) && y.StartsWith( "-" ))              {                  return  "-"  + Add(x.TrimStart( '-' ), y.TrimStart( '-' ));              }              if  (x.Length > y.Length)              {                  y = y.PadLeft(x.Length,  '0' );              }              else              {                  x = x.PadLeft(y.Length,  '0' );              }              int [] sum =  new  int [x.Length + 1];              for  ( int  i = x.Length - 1; i >= 0; i--)              {                  int  tmpsum =  int .Parse(x[i].ToString()) +  int .Parse(y[i].ToString()) + sum[i + 1];                  if  (tmpsum >= 10)                  {                      sum[i + 1] = tmpsum - 10;                      sum[i] = 1; //表示进位                  }                  else                  {                      sum[i + 1] = tmpsum;                  }              }              string  returnvalue =  string .Concat(sum);              if  (sum[0] == 1)              {                  return  returnvalue;              }              else              {                  return  returnvalue.Remove(0, 1);              }          }          //字符串模拟减法操作          static  string  Subtract( string  x,  string  y)          {              //if (x.StartsWith("-") && !y.StartsWith("-"))              //{              //    return "-" + Add(x.TrimStart('-'), y);              //}              //if (y.StartsWith("-"))              //{              //    return Add(x, y.TrimStart('-'));              //}              //x是正数，y也是正数              int  flag = checkBigger(x, y);              if  (flag == 0)              {                  return  "0" ;              }              else  if  (flag == -1)              {                  string  tmp = y;                  y = x;                  x = tmp;              }              //保证了x>=y              y = y.PadLeft(x.Length,  '0' ); //y补0与x对齐              int [] difference =  new  int [x.Length];              for  ( int  i = x.Length - 1; i >= 0; i--)              {                  int  tmpdifference;                  tmpdifference =  int .Parse(x[i].ToString()) -  int .Parse(y[i].ToString()) + difference[i];                  if  (tmpdifference < 0)                  {                      tmpdifference += 10;                      difference[i - 1] = -1; //表示借位                  }                  difference[i] = tmpdifference;              }              StringBuilder returnvalue =  new  StringBuilder( string .Concat(difference).TrimStart( '0' ));              {                  if  (returnvalue.ToString() ==  "" )                  {                      return  "0" ;                  }              }              if  (flag == -1)              {                  returnvalue = returnvalue.Insert(0,  "-" );              }              return  returnvalue.ToString();          }          //比较大小          static  int  checkBigger( string  x,  string  y)          {              if  (x.Length > y.Length)              {                  return  1;              }              else  if  (x.Length < y.Length)              {                  return  -1;              }              else              {                  for  ( int  i = 0; i < x.Length; i++)                  {                      if  ( int .Parse(x[i].ToString()) >  int .Parse(y[i].ToString()))                      {                          return  1;                      }                      else  if  ( int .Parse(x[i].ToString()) <  int .Parse(y[i].ToString()))                      {                          return  -1;                      }                      continue ;                  }                  return  0;              }          }          //模拟移位          static  string  Power10( string  num,  int  n)          {              return  num.PadRight(num.Length + n,  '0' );          }                                             } }