Reverse a String In-Place

Code

#include <stdio.h>

void reverse_string(char *str) {

    int null_index = 0;

    while (str[null_index] != '\0') {
        null_index++;
    }

    for (int i = 0; i <= (null_index-1)/2; i++) {
        char tmp = str[i];
        str[i] = str[null_index-1-i];
        str[null_index-1-i] = tmp;
    }    
}

int main() {
    char str[101];
    fgets(str, sizeof(str), stdin);

    // Remove newline
    int i = 0;
    while (str[i] != '\0') {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }

    reverse_string(str);
    printf("%s", str);
    return 0;
}

Solving Approach