HDU 5805 - NanoApe Loves Sequence （BestCoder Round #86）

先找相邻差值的最大，第二大，第三大

删去端点会减少一个值， 删去其余点会减少两个值，新增一个值，所以新增和现存的最大的值比较一下取最大即可

 #include <iostream>
 #include <cstdio>
 #include <cmath>
 using namespace std;
 #define LL long long
 const int N = ;
 int t, n, p1, p2, p3;
 LL a[N];
 LL s1[N], s2[N];
 LL sum;
 int main()
 {
     scanf("%d", &t);
     while(t--)
     {
         scanf("%d", &n);
         for(int i = ; i <= n; i++)
             scanf("%I64d", &a[i]);
         p1 = p2 = p3 = ;
         s1[] = -;
         for(int i = ; i < n; i++)
         {
             s1[i] = abs(a[i + ] - a[i]);//相邻
             if(s1[p1] < s1[i] ) p1 = i;//最大
         }
         for(int i = ; i < n; i++)
         {
             if(i == p1) continue;
             else if(s1[p2] < s1[i]) p2 = i;//第二大
         }
         for(int i = ; i < n; i++)
         {
             if(i == p1 || i == p2) continue;//第三大
             else if(s1[p3] < s1[i]) p3 = i;
         }
         for(int i = ; i < n; i++)
             s2[i] = abs(a[i+] - a[i-]);//去掉 i 点新增值 

         sum = ;
         if(p1 == ) sum += s1[p2];
         else sum += s1[p1];
         for(int i = ; i < n; i++)
         {
             if(p1 == i- || p1 == i)
             {
                 if(p2 == i- || p2 == i) sum += max(s1[p3], s2[i]);
                 else sum += max(s1[p2], s2[i]);
             }
             else sum += max(s1[p1], s2[i]);
         }
         if(p1 == n - ) sum += s1[p2];
         else sum += s1[p1];
         printf("%I64d\n",sum);
     }
 }