```
class Solution(object):
def findTheDifference(self, s, t):
"""
:type s: str
:type t: str
:rtype: str
"""
l = len(s)
a = []
c = 0
for i in range(l+1):
if t[i] not in s:
a.append(t[i])
else:
c += 1
if c == l+1:
c1 = dict()
c2 = dict()
for s1 in s:
c1[s1] = c1.get(s1,0)+1
for s2 in t:
c2[s2] = c2.get(s2,0)+1
lst1 = list()
for key,val in c1.items():
lst1.append((key,val))
lst2 = list()
for key,val in c2.items():
lst2.append((key,val))
lst1.sort()
lst2.sort()
for j in range(len(lst1)):
if lst1[j][1] != lst2[j][1]:
a.append(lst2[j][0])
return(a[0])
```