luogu P1063 能量项链

传送门

我才不会说我这个题D了好久

区间dp可是一大骗分利器

要写熟练

如果第一层循环要枚举长度真的不如记搜

Time cost: 25min

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
#define ms(a,b) memset(a,b,sizeof a)
#define rep(i,a,n) for(int i = a;i <= n;i++)
#define per(i,n,a) for(int i = n;i >= a;i--)
#define inf 2147483647
using namespace std;
typedef long long ll;
ll read() {
    ll as = 0,fu = 1;
    char c = getchar();
    while(c < '0' || c > '9') {
        if(c == '-') fu = -1;
        c = getchar();
    }
    while(c >= '0' && c <= '9') {
        as = as * 10 + c - '0';
        c = getchar();
    }
    return as * fu;
}
const int N = 105;
//head

int n;
int a[N<<1];
int dp[N<<1][N<<1],maxx;
//[l,r)
int dfs(int l,int r) {
    if(r - l < 2) return 0;
    if(dp[l][r]) return dp[l][r];
    rep(i,l+1,r-1) {
        dp[l][r] = max(dp[l][r],dfs(l,i) + dfs(i,r) + a[l] * a[i] * a[r]);
    }
    return dp[l][r];
}

int main() {
    n = read();
    rep(i,1,n) a[i] = read();
    rep(i,1,n) a[i+n] = a[i];
    dfs(1,n<<1);
    rep(i,1,n) maxx = max(maxx,dp[i][i+n]);
    printf("%d
",maxx);
    return 0;
}

循环

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
#define ms(a,b) memset(a,b,sizeof a)
#define rep(i,a,n) for(int i = a;i <= n;i++)
#define per(i,n,a) for(int i = n;i >= a;i--)
#define inf 2147483647
using namespace std;
typedef long long ll;
ll read() {
    ll as = 0,fu = 1;
    char c = getchar();
    while(c < '0' || c > '9') {
        if(c == '-') fu = -1;
        c = getchar();
    }
    while(c >= '0' && c <= '9') {
        as = as * 10 + c - '0';
        c = getchar();
    }
    return as * fu;
}
const int N = 1005;
//head
int n,maxx;
int a[N];
int dp[N][N];
int main() {
    n = read();
    rep(i,1,n) a[i] = a[i+n] = read();
//    rep(i,1,n<<1) printf("%d ",a[i]);puts("");
    rep(i,2,n+1) {
        rep(l,1,n*2-i+1) {
            int r = l+i-1;
            rep(k,l+1,r-1)
                 dp[l][r] = max(dp[l][r],dp[l][k] + dp[k][r] + a[l]*a[k]*a[r]);
//            printf("%d %d %d
",l,r,dp[l][r]);
        }
    }
    maxx = 0;
    rep(i,1,n) maxx = max(maxx,dp[i][n+i]);
    printf("%d
",maxx);
}

不得不说这种简单题样例真的良心