Leetcode-937 Reorder Log Files(重新排列日志文件)

class Solution
{
    public:
        int distinctSubseqII(string S)
        {
            int n = S.size();
            int mod = 1000000007;
            int h[2002],last[2002],dp[2002];
            char a[2002];
            memset(h,0,sizeof(h));
            memset(last,0,sizeof(last));
            memset(dp,0,sizeof(dp));
            for(int i=1; i<=n; ++i)
            {
                a[i]=S[i-1];
                last[i]=h[a[i]];
                h[a[i]]=i;
            }
            for(int i=1; i<=n; ++i)
            {
                if(!last[i]) dp[i]=dp[i-1]*2+1;
                else dp[i]=dp[i-1]+dp[i-1]-dp[last[i]-1];
                if(dp[i]<0) dp[i]+=mod;
                dp[i]%=mod;
            }
            return dp[n];
        }
};