P3382 【模板】三分法

P3382 【模板】三分法
只有一个地方
在这个题当中，f(mid)>f(mid-eps)比f(mid)-f(mid-eps)>eps精度要高

#include <bits/stdc++.h>
#define inf 2333333333333333
#define N 10010
#define p(a) putchar(a)
#define For(i,a,b) for(long long i=a;i<=b;++i)
//by war
//2020.10.13
using namespace std;
long long n;
long double k[N];
long double l,r,mid,eps=1e-7;
void in(long long &x){
    long long y=1;char c=getchar();x=0;
    while(c<'0'||c>'9'){if(c=='-')y=-1;c=getchar();}
    while(c<='9'&&c>='0'){ x=(x<<1)+(x<<3)+c-'0';c=getchar();}
    x*=y;
}
void o(long long x){
    if(x<0){p('-');x=-x;}
    if(x>9)o(x/10);
    p(x%10+'0');
}

long double f(long double x){
    long double r=0;
    For(i,0,n) r=r*x+k[i];
    return r;
}

signed main(){
    cin>>n>>l>>r;
    For(i,0,n) cin>>k[i];
    while(r-l>=eps){
        mid=(l+r)/2.0;
        if(f(mid)>f(mid-eps)) l=mid;
        else r=mid;
    }
    printf("%.5Lf\n",mid);
    return 0;
}