123 -> 321

#include <stdio.h>
#define length 81
#include <stdlib.h>
#include <string.h>
char *reverse(char *mass, int n) {
    if (n != 0) {
        int i = 0;
        int j = n - 1;
        for ( ; i < j; i++, j--) {
            char c = *(mass + i);
            *(mass + i) = *(mass + j);
            *(mass + j) = c;
        }
    }

    return mass;

}
int main()
{
    char s[length];
    int n = 0, i = 0;
    printf("Введите строку\n");
    gets_s(s);
    while (s[i] != '\0') {
        n++;
            i++;
    }
    reverse(s, n);
    printf("%s\n", s);
    system("pause");
    return 0;
}