Write a function find with two parameters: str and substr. The parameters str and substr are pointers to char. The function find returns the address of the first occurrence of the string pointed by substr in the string pointed to by str. If the string pointed to by substr does not occur in the string pointed to by str, find returns Null. Example: If str points to "Let's talk about the black bird." and substr points to "black", then find returns a pointer to the character b (in black) in the string pointed to by str. If str points to "Let's talk about the black bird." and substr points to "blue", then find returns the value. NULL. Write a main function that invokes find several times to demonstrate that it is working properly. You cannot use string handling function such as strstr for this problem. The problem actually asks you to implement strstr by yourself.