poj3670

O(n)的dp

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#include <iostream>

#include <cstdio>

#include <cstdlib>

#include <cstring>

using namespace std;



#define maxn 30005



int n;

int cow[maxn];

int f[maxn][5];



void input()

{

    scanf("%d", &n);

    for (int i = 1; i <= n; i++)

        scanf("%d", &cow[i]);

}



void work()

{

    memset(f, 0, sizeof(f));

    for (int i = 1; i <= n; i++)

    {

        for (int j = 1; j <= 3; j++)

        {

            f[i][j] = f[i - 1][1];

            for (int k = 2; k <= j; k++)

                f[i][j] = min(f[i][j], f[i - 1][k]);

            if (cow[i] != j)

                f[i][j]++;

//            printf("%d %d %d\n", i, j, f[i][j]);

        }

    }

    int ans = n;

    for (int i = 1; i <= 3; i++)

        ans = min(ans, f[n][i]);

    for (int i = 1; i <= n; i++)

    {

        for (int j = 1; j <= 3; j++)

        {

            f[i][j] = f[i - 1][3];

            for (int k = 2; k >= j; k--)

                f[i][j] = min(f[i][j], f[i - 1][k]);

            if (cow[i] != j)

                f[i][j]++;

        }

    }

    for (int i = 1; i <= 3; i++)

        ans = min(ans, f[n][i]);

    printf("%d\n", ans);

}



int main()

{

    //freopen("t.txt", "r", stdin);

    input();

    work();

    return 0;

}