1 class Solution
 2 {
 3     public:
 4         int distinctSubseqII(string S)
 5         {
 6             int n = S.size();
 7             int mod = 1000000007;
 8             int h[2002],last[2002],dp[2002];
 9             char a[2002];
10             memset(h,0,sizeof(h));
11             memset(last,0,sizeof(last));
 
12             memset(dp,0,sizeof(dp));
13             for(int i=1; i<=n; ++i)
14             {
15                 a[i]=S[i-1];
16                 last[i]=h[a[i]];
17                 h[a[i]]=i;
18             }
19             for(int i=1; i<=n; ++i)
20             {
21                 if(!last[i]) dp[i]=dp[i-1
 
]*2+1;
22                 else dp[i]=dp[i-1]+dp[i-1]-dp[last[i]-1];
23                 if(dp[i]<0) dp[i]+=mod;
24                 dp[i]%=mod;
25             }
26             return dp[n];
27         }
28 };