题解 | #abb#
abb
https://www.nowcoder.com/practice/0a8bbf8b9b5b4280957849ef4f240f07
倒着搞会方便一点
n=int(input())
s=input()
d=[0]*128
ans=0
for c in s[::-1]:
x=ord(c)
for i in range(97,123):
if(i==x):
d[i]+=1
else:
ans+=d[i]*(d[i]-1)//2
print(ans)