```
class Solution {
public:
char findTheDifference(string s, string t) {
int s_len = s.length();
int t_len = t.length();
const int n = 26;
vector<int> cnt(n, 0);
for (int i = 0; i < s_len; i++)
{
cnt[s[i] - 'a']++;
}
for (int j = 0; j < t_len; j++)
{
cnt[t[j] - 'a']--;
}
for (int k = 0; k < n; k++)
{
if (cnt[k] < 0)
return k + 'a';
}
return 'a';
}
};
```