PAT T1020 Delete At Most Two Characters

二维dp，i表示删除几个字符，j表示当前遍历到第几个字符，注意判重复~

#include<bits/stdc++.h>
using namespace std;
const int maxn=1e6+14;
string s;
long long dp[5][maxn];
int main () {
    getline(cin,s);
    fill (dp[0],dp[0]+maxn,1);
    for (int i=1;i<=2;i++) {
        for (int j=i;j<=s.length();j++) {
            dp[i][j]=dp[i][j-1]+dp[i-1][j-1];
            for (int k=j-1;j-k<=i;k--) 
            if (s[k-1]==s[j-1]) {
                dp[i][j]-=dp[i-(j-k)][k-1];
                break;
            }
        } 
    }
    printf ("%lld",dp[0][s.length()]+dp[1][s.length()]+dp[2][s.length()]);
    return 0;
}