cc150.1.2.c

#include <stdio.h>
#include <string.h>

//1.2 Implement a function void reverse(char* str) in C or C++
// which reverses a null terminated string.

//O(N) time, O(1) space
void reverse(char* str) {
    if (str == NULL) {
        return;
    }

    unsigned long length = strlen(str);

    /* here is what starlen does
    while (*(str + length)) {
        ++length;
    }
    */

    char *head = str;
    char *tail = str + length -1;
    while ( tail - head > 0 ) { //will handle even and odd and 0 length situation 
        char temp = *head;
        *head = *tail;
        *tail = temp;
        ++head;
        --tail;
    }
}

int main() {    
    char t1[100]; //can't use string literal here, because you can't modify it
    //otherwise you'll get "Bus error 10" error, ask me how I know!
    reverse(NULL);
    
    strcpy(t1, "");
    printf("before:\"%s\"\n", t1);
    reverse(t1);
    printf("after:\"%s\"\n", t1);

    strcpy(t1, "a");
    printf("before:\"%s\"\n", t1);
    reverse(t1);
    printf("after:\"%s\"\n", t1);

    strcpy(t1, "ab");
    printf("before:\"%s\"\n", t1);
    reverse(t1);
    printf("after:\"%s\"\n", t1);

    strcpy(t1, "abc");
    printf("before:\"%s\"\n", t1);
    reverse(t1);
    printf("after:\"%s\"\n", t1);

    strcpy(t1, "abcd");
    printf("before:\"%s\"\n", t1);
    reverse(t1);
    printf("after:\"%s\"\n", t1);
}