Q.

Description:
Simple, remove the spaces from the string, then return the resultant string.

For C, you must return a new dynamically allocated string.

A)

#include <stdlib.h>

char *no_space(const char *str_in)
{
  char *ptr;
  int i = 0, j = 0, len = 0;
  while (str_in[i])
  {
    if (str_in[i] != ' ')
      len++;
    i++;
  }
  if (!(ptr = (char*)malloc(sizeof(char) * len + 1)))
    return NULL;
  i = 0;
  while (str_in[i])
  {
    if (str_in[i] != ' ')
      ptr[j++] = str_in[i];
    i++;
  }
  ptr[len] = '\0';
  return ptr;
}

another solution
char *no_space(char *s) {
  char *res = strdup(s), *q = res;
  for (; *s; s++) 
  	if (*s != ' ') 
    	*q++ = *s;
  return *q = 0, res;
} -> strdup() function : 문자열 동적할당해주고 카피해주는데 이렇게 하면 직접 쓸 메모리공간보다 더 할당해주기 때문에 낭비될 수 있음.