bzoj4760 [Usaco2017 Jan]Hoof, Paper, Scissors

传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=4760

【题解】

直接dp，f[i][j][k]表示到了第i个，用了j次改变，上一次出k。

直接转移，压下空间。

# include <stdio.h>
# include <string.h>
# include <iostream>
# include <algorithm>
// # include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int M = 5e5 + 10;
const int mod = 1e9+7;

# define RG register
# define ST static

int n, K;
char opt[3];
int a[M];
// 0    1    2
// 蹄子 剪刀 布 
int f[2][23][3];

int main() {
    cin >> n >> K;
    for (int i=1; i<=n; ++i) {
        scanf("%s", opt);
        if(opt[0] == 'P') a[i] = 2;
        else if(opt[0] == 'H') a[i] = 0;
        else a[i] = 1;
    }
    int pre = 0, cur = 1;
    for (int i=1; i<=n; ++i) {
         for (int j=0; j<=K; ++j) {
             for (int k=0; k<=2; ++k) {
                 for (int lst=0; lst<=2; ++lst) {
                     if(k == lst) f[cur][j][k] = max(f[cur][j][k], f[pre][j][lst]);
                     else {
                        if(j) f[cur][j][k] = max(f[cur][j][k], f[pre][j-1][lst]);
                    }
                }
                f[cur][j][k] += (a[i] == (k+1)%3);
            }
        }
        swap(cur, pre);
    }
    int ans = 0;
    for (int j=0; j<=K; ++j)
        for (int k=0; k<=2; ++k) 
            ans = max(ans, f[pre][j][k]);
    printf("%d
", ans);
    return 0;
}