This problem is similar to https://leetcode.com/problems/single-number/description/

In both cases, we use XOR or cancel elements out with each other.

'a' XOR 'a' == 97 XOR 97 == 0

```
public class Solution {
public char findTheDifference(String s, String t) {
int xor = 0;
for(int i = 0; i < s.length(); i++) xor ^= s.charAt(i);
for(int i = 0; i < t.length(); i++) xor ^= t.charAt(i);
return (char)xor;
}
}
```