Here we will use a helper array of 26 numbers.

now we count all the chars and put it into its own cell

example "aaabccddddde" will be something like

3 1 2 5 1 ... that corresponds to

a b c d e ...

then, having the count of chars, we simply go over string t and deduct now similar way.

if the second string is "aaabcddddde" (missing one c) you will have leftovers as

0 0 1 0 0 ... that corresponds to

a b c d e ...

Now we simply find that +1 or -1 - that will be our char

```
char findTheDifference(string s, string t)
{
vector<int> chMap(26, 0);
// count every char for string s
for (int i=0; i<s.size(); ++i)
{
chMap[s[i]-'a'] += 1;
}
// now substitute every char from t
for (int i=0; i<t.size(); ++i)
{
chMap[t[i]-'a'] -=1;
}
// now we should have whether +1/-1 and all the rest 0
for (int i=0; i<26; ++i)
{
if (chMap[i] != 0)
return 'a' + i;
}
return 0;
}
```