BZOJ 5324: [Jxoi2018]守卫 区间DP

思路非常巧妙啊 

code:

#include <cstdio> 
#include <algorithm>  
#define ll long long 
#define N 5003 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;
int f[N][N],h[N],n;   
double slope(int l,int r){return (double)(h[r]-h[l])/(r-l);}
bool cansee(int l,int x,int r){return slope(x,r)>slope(l,r);}
int main() 
{ 
    // setIO("input"); 
    int i,j,ans=0;   
    scanf("%d",&n); 
    for(i=1;i<=n;++i) scanf("%d",&h[i]);   
    for(int r=1;r<=n;++r) 
    {
        f[r][r]=1; 
        ans^=f[r][r];   
        int sum=1,p=0;   
        for(int l=r-1;l>=1;--l) 
        { 
            if(!p||cansee(l,p,r)) 
            {
                sum+=min(f[l+1][p-1],f[l+1][p]);    
                p=l;  
            }
            f[l][r]=sum+min(f[l][p],f[l][p-1]);   
            ans^=f[l][r];   
        }
    }
    printf("%d
",ans);  
    return 0;
}