字符串替换--替换字符串中的空格为$$$。要求时间复杂度为O(N)

char* Repalce (char * str )
{
int blank = 0;
int newlen = 0;
int oldlen = strlen( str);
for ( int i = 0; i < oldlen; ++ i)
{
if (*( str + i) == ' ')
blank++;
}

newlen = oldlen + 2 * blank;
char* newend = str + newlen;
char* oldend = str + oldlen;

while ( oldend < newend)
{
if (* oldend == ' ')
{
for ( int i = 1; i <= 3;++ i)
* newend-- = '$';
oldend--;
}
else{
* newend-- = * oldend--;
}
}
return str;
}