二分和三分

1.二分 NOIP2016 跳石头

//Twenty
#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn=50000+100;
int l1,m,n,a[maxn],b[maxn],l,r,mid,ans,mxx=1e9;

int check(int ans)
{
    for(int i=1;i<=n;i++)
    b[i]=a[i];
    
    int st=m;
    for(int i=1;i<=n;i++)
    {
        if(a[i]-b[i-1]<ans)
        {
           st--;
           if(st<0)return 0;
        b[i]=a[i-1];
        }
    }
    return 1;
}

int main()
{
    freopen("stone.in","r",stdin);
    freopen("stone.out","w",stdout);
    scanf("%d%d%d",&l1,&n,&m);
    for(int i=1;i<=n;i++)
    {
    scanf("%d",&a[i]);
    if(a[i]<mxx)mxx=a[i];
    }
    a[0]=0;
    a[++n]=l1;
    l=mxx;r=l1;
    while(l<=r)
    {
       mid=(l+r)>>1;
       if(check(mid)){ans=mid;l=mid+1;}
       else r=mid-1;        
    }
    printf("%d
",ans);
    return 0;
}

2.洛谷三分模板

//Twenty
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<queue>
#include<cmath>
using namespace std;
const double eps=1e-7;
int n;
double ans,a[20],l,r;
double js(double x){
    double base=1,ret=0;
    for(int i=n;i>=1;i--){
        base*=x;
        ret+=base*a[i];
    }
    return ret+a[n+1];
}
int main()
{
    scanf("%d%lf%lf",&n,&l,&r);
    for(int i=1;i<=n+1;i++)
    scanf("%lf",&a[i]);
    while(r-l>eps){
        double mid1=l+(r-l)/3,mid2=r-(r-l)/3;
        if(js(mid1)>=js(mid2)) ans=mid1,r=mid2;
        else l=mid1;
    }
    printf("%.5lf",l);
    return 0;
}