4题题解来喽~ #include<bits/stdc++.h> #define int long long using namespace std; int const N=1e6+10; string s; int f[N],f1[N]; signed main(){ cin>>s; map<char,int>mp,mp1; for (int i=0;i<s.size();i++){ if(mp.count(s[i])){ f[i]=mp[s[i]]; f1[i]+=mp1[s[i]]; } mp[s[i]]++; mp1[s[i]]+=i; } int ans=0; for (int i=0;i<s.size();i++){ int j=i; ans+=f[i]*(i-1)-f1[i]; } cout<<ans; return 0; }