[ZJOI2006]皇帝的烦恼

#include <bits/stdc++.h>
using namespace std;
#define rint register int
#define IL inline
#define rep(i,h,t) for (rint i=h;i<=t;i++)
#define dep(i,t,h) for (rint i=t;i>=h;i--)
#define mid ((h+t)>>1)
const int N=2e5;
int n,m,a[N],mx[N],nx[N];
bool check(int x)
{
  mx[1]=a[1],nx[1]=a[1];
  rep(i,2,n)
  {
    mx[i]=min(a[1]-nx[i-1],a[i]);
    nx[i]=max(0,a[i]-((x-a[i-1])-(a[1]-mx[i-1])));
  }
  if (nx[n]==0) return(1);
  else return(0);
}
int main()
{
  freopen("1.in","r",stdin);
  freopen("1.out","w",stdout);
  ios::sync_with_stdio(false);
  int maxa=0;
  cin>>n;
  rep(i,1,n) cin>>a[i];
  rep(i,1,n-1) maxa=max(maxa,a[i]+a[i+1]);
  int h=maxa,t=1e9;
  while (h<t)
  {
    if (check(mid)) t=mid; else h=mid+1;
  }
  cout<<h<<endl;
  return 0; 
}