Write a C program that takes a string input from the user and shifts each character forward by one using pointer arithmetic. For instance, the string "Hello" should be changed to "Ifmmp". 

Convert this program into a function called encode(char arr[]) and implement another function called decode(char arr[]) to shift each character of a string back by one. 

char str[] = "Hello";
encode(str); /* str changed to "Iffmp" */
decode(str); /* str changed to "Hello" */