HDU 4455 Substrings ( DP好题 )

这个……真心看不出来是个DP，我在树状数组的康庄大道上欢快的奔跑了一下午……看了题解才发现错的有多离谱。

参考：http://www.cnblogs.com/kuangbin/archive/2012/11/11/2765329.html

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>

#define LL long long int

using namespace std;

const int MAXN = 1000100;

int val[MAXN];
int pos[MAXN];
int cnt[MAXN];
int diff[MAXN];
bool vis[MAXN];
LL dp[MAXN];
int N;

int main()
{
    while ( scanf( "%d", &N ) == 1 && N )
    {
        memset( cnt, 0, sizeof(int)*(N+4) );
        memset( pos, 0, sizeof(int)*(N+4) );

        for ( int i = 1; i <= N; ++i )
        {
            scanf( "%d", &val[i] );
            ++cnt[ i - pos[ val[i] ] ];
            pos[ val[i] ] = i;
        }

        memset( vis, false, sizeof(bool)*(N+4) );
        diff[1] = 1;
        vis[ val[N] ] = true;
        for ( int i = 2; i <= N; ++i )
        {
            if ( vis[ val[N-i+1] ] )
                diff[i] = diff[i - 1];
            else
            {
                diff[i] = diff[i - 1] + 1;
                vis[ val[N-i+1] ] = true;
            }
        }

        int tot = N;
        dp[1] = N;
        for ( int i = 2; i <= N; ++i )
        {
            dp[i] = dp[i - 1] - diff[i - 1];
            tot -= cnt[i - 1];
            dp[i] += tot;
        }

        int Q;
        scanf("%d", &Q );
        while ( Q-- )
        {
            int w;
            scanf("%d", &w);
            printf("%I64d
", dp[w] );
        }
    }
    return 0;
}