1020 Delete At Most Two Characters (35 分)

include<bits/stdc++.h>
using namespace std;
const int N = 1e6+5;
typedef long long ll;
ll dp[3][N]={0};
char s[N];
int main(){
    scanf("%s",s+1);
    int len = strlen(s+1);
    // cout<<len<<endl;
    ll sum = 0;
    for(int i=0;i<=2;i++){
        for(int j=i;j<=len;j++){
            if(i==0) dp[i][j] = 1;
            else{
                dp[i][j] = dp[i][j-1] + dp[i-1][j-1];
                for(int k=j-1;;k--){
                    if(j-k > i) break;
                    if(s[k] == s[j]){
                        dp[i][j] -= dp[i-(j-k)][k-1];
                        break;
                    }
                }
            }
        }
    }
    sum = dp[0][len]+dp[1][len]+dp[2][len];
    cout<<sum<<endl;   
}