BZOJ 1609 麻烦的聚餐

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 30050
#define inf 2000000000
using namespace std;
int n,a[maxn],ans=inf,dp[maxn][4];
void pre()
{
    for (int i=1;i<=n;i++) dp[i][1]=dp[i][2]=dp[i][3]=inf;
    for (int i=1;i<=3;i++)
    {
        if (i==a[1]) dp[1][i]=0;
        else dp[1][i]=1;
    }
}
int main()
{
    scanf("%d",&n);
    for (int i=1;i<=n;i++) scanf("%d",&a[i]);
    pre();
    for (int i=2;i<=n;i++)
    {
        dp[i][1]=dp[i-1][1];if (a[i]!=1) dp[i][1]++;
        dp[i][2]=min(dp[i-1][1],dp[i-1][2]);if (a[i]!=2) dp[i][2]++;
        dp[i][3]=min(min(dp[i-1][1],dp[i-1][2]),dp[i-1][3]);if (a[i]!=3) dp[i][3]++;
    }
    ans=min(ans,min(dp[n][1],min(dp[n][2],dp[n][3])));
    pre();
    for (int i=2;i<=n;i++)
    {
        dp[i][1]=min(min(dp[i-1][1],dp[i-1][2]),dp[i-1][3]);if (a[i]!=1) dp[i][1]++;
        dp[i][2]=min(dp[i-1][2],dp[i-1][3]);if (a[i]!=2) dp[i][2]++;
        dp[i][3]=dp[i-1][3];if (a[i]!=3) dp[i][3]++;
    }
    ans=min(ans,min(dp[n][1],min(dp[n][2],dp[n][3])));
    printf("%d\n",ans);
    return 0;
}