hdu 5135

？？？：

#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cmath>
using namespace std;

double a[15];
double dp[5][15];

bool isOk(double a, double b, double c)
{
    if((a+b)>c && (a+c)>b && (b+c)>a)
        return true;
    return false;
}
double area(double a, double b, double c){
    double x = (a+b+c)/2.0;
    double s =sqrt(x*(x-a)*(x-b)*(x-c));
    return s;
}
int main(){
    int n;
    while(~scanf("%d",&n)){
        memset(dp,0,sizeof dp);
        if(n == 0)
            break;
        for(int i = 0; i < n; i++)
            scanf("%lf",&a[i]);
        sort(a,a+n);
        int m = n/3;
        for(int i = 0; i < m; i++)
            for(int j = 2; j < n; j++){
                dp[i][j] = dp[i][j-1];
                if(isOk(a[j-2],a[j-1],a[j]))
                {
                    if(j != 2)
                        dp[i][j] = max(dp[i][j],(area(a[j-2],a[j-1],a[j]) + dp[i-1][j-3]));
                    else
                        dp[i][j] = area(a[j-2],a[j-1],a[j]);
                }
            }
        printf("%.2lf\n",dp[m-1][n-1]);
    }
    return 0;
}

贪心：

#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <cmath>
using namespace std;

double a[15];
double dp[5][15];

bool isOk(double a, double b, double c)
{
    if((a+b)>c && (a+c)>b && (b+c)>a)
        return true;
    return false;
}
double area(double a, double b, double c){
    double x = (a+b+c)/2.0;
    double s =sqrt(x*(x-a)*(x-b)*(x-c));
    return s;
}
int main(){
    int n;
    while(~scanf("%d",&n)){
        memset(dp,0,sizeof dp);
        if(n == 0)
            break;
        for(int i = 0; i < n; i++)
            scanf("%lf",&a[i]);
        sort(a,a+n);
        double sum = 0.0;
        for(int i = n-3; i >= 0; i--){
            if(isOk(a[i],a[i+1],a[i+2])){
                sum += area(a[i],a[i+1],a[i+2]);
                i = i-2;
            }
        }
        printf("%.2lf\n",sum);
    }
    return 0;
}

状态压缩不会 http://blog.youkuaiyun.com/yeyeyeguoguo/article/details/47859671