学步园

分奇偶讨论；

#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <vector>
#include <map>
using namespace std;
typedef long long LL;

const int maxn = 100000+10;
int n,a[maxn],le[maxn],ri[maxn];
int Judge(int x){
  le[1] = a[1]; ri[1] = 0;
  int p = a[1];
  for(int i=2;i<=n;i++){
    if(i&1){
        ri[i]=min(x-p-ri[i-1],a[i]);
        le[i]=a[i]-ri[i];
    }
    else {
        le[i]=min(p-le[i-1],a[i]);
        ri[i]=a[i]-le[i];
    }
  }
  return le[n]==0;
}
int main()
{
    while(scanf("%d",&n)==1 && n){
      int x = 0;
      for(int i=1;i<=n;i++){
        scanf("%d",&a[i]);
      }
      for(int i=1;i<=n;i++){
        int next = i==n ? 1 : i+1;
        x=max(x,a[i]+a[next]);
      }
      if(!(n&1)){
         printf("%d\n",x);
         continue;
      }
      else {
        if(n==1){
             printf("%d\n",a[1]);
             continue;
        }
        int y = 500000;
        while(x<y){
            int m = x+ (y-x)/2;
            if(Judge(m)) y=m;
            else x=m+1;
        }
        printf("%d\n",x);
      }
    }
    return 0;
}