c里面居然没有取子串的函数,这不是给老师出题的机会吗？
char * substr(char *string, int start , int end){
int len = strlen(string);
    if(end > len) {
  end = len;
}

if(start < 0) {
  start = len + start;
  if(start < 0) {
   start = 0;
  }
}

if(end <= 0) {
  end = len + end;
  if(end < start) {
   end = len;
  }
}
if(start > end || start < 0) {
  start = 0;
}
char * substring = (char *) malloc(strlen(string)+1);
strncpy(substring, &string[start], end);
return substring;
}

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