# P2885 [USACO07NOV]电话线Telephone Wire

## 题目描述

Farmer John's cows are getting restless about their poor telephone service; they want FJ to replace the old telephone wire with new, more efficient wire. The new wiring will utilize N (2 ≤ N ≤ 100,000) already-installed telephone poles, each with some heighti meters (1 ≤ heighti ≤ 100). The new wire will connect the tops of each pair of adjacent poles and will incur a penalty cost C × the two poles' height difference for each section of wire where the poles are of different heights (1 ≤ C ≤ 100). The poles, of course, are in a certain sequence and can not be moved.

Farmer John figures that if he makes some poles taller he can reduce his penalties, though with some other additional cost. He can add an integer X number of meters to a pole at a cost of X2.

Help Farmer John determine the cheapest combination of growing pole heights and connecting wire so that the cows can get their new and improved service.

## 输入输出格式

• Line 1: Two space-separated integers: N and C
• Lines 2..N+1: Line i+1 contains a single integer: heighti

• Line 1: The minimum total amount of money that it will cost Farmer John to attach the new telephone wire.

## 输入输出样例

```5 2
2
3
5
1
4```

```一开始自己写了个DP，居然能过样例

``` 1 #include<iostream>
2 #include<cstdio>
3 #include<cstring>
4 #include<cmath>
5 #include<cstring>
6 #include<algorithm>
7 #include<queue>
8 #include<cstdlib>
9 using namespace std;
10 const int MAXN=100001;
11 const int INF =0x7f7f7f7f;
13 {
14     char c='+';bool flag=0;n=0;
15     while(c<'0'||c>'9'){c=getchar();if(c=='-')flag=1;}
16     while(c>='0'&&c<='9')n=n*10+c-48,c=getchar();
17 }
18 int dp[MAXN][201];// 第i棵树，高度为j的最小花费
19 int n,C;
20 int a[MAXN];
21 int maxheight;
22 int main()
23 {
25     memset(dp,INF,sizeof(dp));
26     for(int i=1;i<=n;i++)
28     for(int i=a[1];i<=maxheight;i++)
29         dp[1][i]=(i-a[1])*(i-a[1]);
30
31     for(int i=2;i<=n;i++)//枚举所有树
32         for(int j=a[i];j<=maxheight;j++)//枚举这棵树的高度
33             for(int k=a[i-1];k<=maxheight;k++)//枚举前一棵树的高度
34                 dp[i][j]=min(dp[i][j],
35                             ((j-a[i])*(j-a[i])+(dp[i-1][k])+abs(j-k)*C));
36
37
38     int ans=0x7fffff;
39     for(int i=a[n];i<=maxheight;i++)
40         ans=min(ans,dp[n][i]);
41     printf("%d",ans);
42     return 0;
43 }```
`然后化简一下式子`
``` 1 #include<iostream>
2 #include<cstdio>
3 #include<cstdlib>
4 #include<cstring>
5 using namespace std;
6 const int MAXN=300005;
7 const int INF =0x7fffff;
8 const int maxheight=100;
9 int dp[301];// 第i棵树，高度为j的最小花费
10 int f[301];
11 int n,C;
12 int a[MAXN];
13 int bgsum[MAXN];
14 int edsum[MAXN];
15 int main() {
16     scanf("%d%d",&n,&C);
17     for(int i=0; i<n; i++)
18         scanf("%d",&a[i]);
19     memset(dp, 0x3f, sizeof(dp));
20     for(int i=a[0]; i<=maxheight; i++)
21         dp[i]=(i-a[0])*(i-a[0]);
22
23     for(int i=1; i<n; i++) { //枚举所有树
24         memcpy(f,dp,sizeof(dp));
25         for(int j=0; j<=maxheight; j++)    dp[j]=bgsum[j]=edsum[j]=INF;
26
27         bgsum[0]=f[0];
28         for(int j=1; j<=maxheight; j++)
29             bgsum[j]=min(bgsum[j-1],f[j]-C*j);
30
31         edsum[maxheight]=f[maxheight]+maxheight*C;
32         for(int j=maxheight-1; j>=0; j--)
33             edsum[j]=min(edsum[j+1],f[j]+C*j);
34
35         for(int j=a[i]; j<=maxheight; j++) //枚举这棵树的高度
36             dp[j]=min(edsum[j]-C*j,bgsum[j]+C*j)+(j-a[i])*(j-a[i]);
37     }
38     int ans=0x7fffff;
39     for(int i=a[n-1]; i<=maxheight; i++)
40         ans=min(ans,dp[i]);
41     printf("%d",ans);
42     return 0;
43 }```

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