Reverse String (C) --Watch out the index

void reverseString(char * str)

• Arguments:
  char * str -- a pointer to a string

• Description:
  Reverses the given string str. The function preserves letter cases and white spaces. For example, the reverse string of "the" is "eht", and "Here I am" is "ma I ereH".

Version 1:

void reverseString(char * str)
{
    if (str == NULL) return;
    char *sp = str;
    int len = 0;
    while ( *sp != '\0') {
        len++;
        sp++;
    }

    char copy[len];
    int i,j;
    // be careful of the index
    // the index of last char in str is (len - 1)
    for (i = 0, j = len - 1; i < len; i++, j--) {
        copy[i] = str[j];
    }
    
    for (i = 0; i < len; i++, str++) {
        *str = copy[i];
    }
}

Version 2:
After we know the length of the string, we can swap the first half and the second half directly.
We can make the solution a little better. Though it still takes O(n) time because of the first loop.

void reverseString(char * str)
{
    if (str == NULL) return;
    char *sp = str;
    int len = 0;
    while ( *sp != '\0') {
        len++;
        sp++;
    }

    int i, j;
    /*
     * we only need to iterate to the middle of the string
     * len / 2 takes care of both odd and even cases
     */
    for (i = 0, j = len - 1; i < len / 2; i++, j--) {
        char temp = str[i];
        str[i] = str[j];
        str[j] = temp;
    }
    
}