重写strstr、strcpy、memcpy、memset、atof算法

 #include<stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <ctype.h>
 #include <math.h>

 char *Mystrstr(const char *string, const char *strCharSet)
 {
     if (NULL == string)
     {
         return (char *)string;
     }
     if (NULL == strCharSet)
     {
         return NULL;
     }
     int nLen1 = strlen(string);
     int nLen2 = strlen(strCharSet);

     for (int i = ; i < nLen2; i++)
     {
         for (int j = ; j <nLen1; j++)
         {
             if (strCharSet[i] == string[j])
             {
                 int nPos = j;
                 while ( (i < nLen2) &&(strCharSet[i] == string[j]))
                 {
                     i++;
                     j++;
                 }
                 if (i == nLen2)
                 {
                     return (char *)&string[nPos];
                 }
                 i = nPos;
             }
         }
     }
     return NULL;
 }

 char *Mystrcpy( char *strDestination, const char *strSource )
 {
     if (strDestination == NULL && strDestination == NULL)
     {
         exit();
     }

     while (*strSource != '\0')
     {
         *strDestination++ = *strSource++;
     }
     *strDestination = '\0';
     return strDestination;
 }

 void *Mymemset( void *dest, int c, size_t count )
 {
     char *pszDest = (char *)dest;
     if (pszDest == NULL)
     {
         exit();
     }
     if ( == count)
     {
         return dest;
     }
     for (size_t i = ; i < count; i++)
     {
         *pszDest++ = c;
     }
     return dest;
 }

 void* MyMemcpy( void *dest, const void *src, int nCount )
 {
     if (NULL == dest || src == NULL)
     {
         return NULL;
     }
     char *pszDest = (char *)dest;
     const char *pszSrc = (char *)src;

     if (pszDest > pszSrc && pszDest < pszSrc + nCount)
     {//有重复（pszDest的位置 在pszSrc里面）
         for (int i = nCount - ; i >= ; i--)
         {//从后往前拷贝
             pszDest[i] = pszSrc[i];
         }
     }
     else
     {
         for (int i = ; i < nCount; i++)
         {
             pszDest[i] = pszSrc[i];
         }
     }
     return dest;
 }

 double Myatof( const char *string )
 {
     bool IsNegative = false;
     bool IsInt = true;
     double dblResult = ;
     int i = ;
     while (*string != '\0')
     {
         switch(*string) {
             case ' ':
                 string++;
                 break;
             case '-':
                 IsNegative = true;
                 string++;
                 break;
             case '+':
                 IsNegative = false;
                 string++;
                 break;
             case '.':
                 IsInt = false;
                 string++;
                 break;
             default:
                 if (IsInt)
                 {
                     dblResult = dblResult* + (*string - '');
                     string++;
                 }
                 else
                 {
                     dblResult += (*string - '') / pow(, i++);
                     string++;
                 }
                 break;
          }//end_of_switch

     }//end_of_while

     return IsNegative? -dblResult : dblResult;
 }

 int main()
 {
     char szBuf[];
     memset(szBuf, , sizeof(szBuf));
     for (int i = ; i < ; i++)
     {
         szBuf[i] = 'a' + i;
     }
     MyMemcpy(&szBuf[], &szBuf[], );
     printf("%s\n", szBuf);
     return ;
 }