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 = 0; i < nLen2; i++)
    {
        for (int j = 0; 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(0);
    }
    
    while (*strSource != '')
    {
        *strDestination++ = *strSource++;
    }
    *strDestination = '';
    return strDestination;
}

void *Mymemset( void *dest, int c, size_t count )
{
    char *pszDest = (char *)dest;
    if (pszDest == NULL)
    {
        exit(0);
    }
    if (0 == count)
    {
        return dest;
    }
    for (size_t i = 0; 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 - 1; i >= 0; i--)
        {//从后往前拷贝
            pszDest[i] = pszSrc[i];
        }
    }
    else
    {
        for (int i = 0; i < nCount; i++)
        {
            pszDest[i] = pszSrc[i];
        }
    }
    return dest;
}



double Myatof( const char *string )
{
    bool IsNegative = false;
    bool IsInt = true;
    double dblResult = 0;
    int i = 1;
    while (*string != '')
    {
        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*10 + (*string - '0');
                    string++;
                }
                else
                {
                    dblResult += (*string - '0') / pow(10, i++); 
                    string++;
                }
                break;
         }//end_of_switch
        
    }//end_of_while
    
    return IsNegative? -dblResult : dblResult;
}


int main()
{
    char szBuf[21];
    memset(szBuf, 0, sizeof(szBuf));
    for (int i = 0; i < 20; i++)
    {
        szBuf[i] = 'a' + i;
    }
    MyMemcpy(&szBuf[10], &szBuf[2], 4);
    printf("%s
", szBuf);
    return 0;
}