Find the Difference

Try to solve the Find the Difference problem.

We'll cover the following

Statement

Given two strings, str1 and str2, find the index of the extra character that is present in only one of the strings.

Note: If multiple instances of the extra character exist, return the index of the first occurrence of the character in the longer string.

Constraints:

$0 \leq$ str1.length, str2.length $\leq 1000$
Either str2.length $=$ str1.length + 1, or, str1.length $=$ str2.length + 1
The strings consist of lowercase English letters.