gistfile1.c

/*
* Space complexity: O(1); Time complexity: O(n).
*/
void reverseString(char* str){
  if(str == NULL) return; // Validation checking.
  if(*str == "\0") return; // Empty string.
  
  char temp; // For swapping tail and head.
  
  char* head=str, tail=str; // Pointers for the beginning and the end of the string.
  
  while(*tail != "\0") tail++; // Find the end of the string.
  
  tail--; 
  
  while(head<=tail){ // Swap the string.
    temp = *head;
    *head = *tail;
    *tail = temp;
    head++;
    tail--;
  }

}

Time complexity should be O(n) because your while block loops n/2 times.