Leveling Ground
题目连接：
Description
It is important to first level the ground before you build anything on top of it (e.g., new house, shed,
swimming pool, driveway, etc.), especially when there are hills on the land, otherwise whatever you
built will not be stable. In case you don’t understand, “leveling the ground” means making the ground
flat and even (having the same height). In this problem, you are given a land description and the
length of land — M — that you want to level; your task is to determine the minimum amount of land
you should dispose in order to have a level land of length M. Note that in this problem you are only
allowed to dispose land, not filling it.
The total length of the given land will be N, and the land will be encoded with the following format:
(1) / means ascending slope (disposing an ascending slope cost 0.5),
(2) \ means descending slope (disposing a descending slope cost 0.5),
(3) means flat (disposing a flat land cost 0),
(4) . means full land (disposing a full land cost 1).
Note that the input will only describe the land’s surface, thus (4) will not appear in any input. Also
note that (1) and (2) are not level.
For example, consider the following input.
Input : //_//_\_////\/
The input corresponds to the following land (which length is 31).
__ /_ __
/.._/...\ /..\ _
Land : /..........\ ___ /...._/.
............_/.../...........
...............................
Index : 1234567890123456789012345678901
Supposed we want to level a land of length M = 7, and for some reasons, we choose the land we
want to level to be at index [11, 17]. Recall that you are only allowed to dispose land, thus if you want
to level the land at [11, 17], you should level it such that the height is equal to the height of land at
index 14 (because it is the lowest point). In the following figure, ‘’ (stars) mark the land which should
be disposed.
__ /_ __ __ /_ __
/.._/... /..\ _ /.._/...| /..\ _
/.........** **_ /...._/. /.........| _ /...._/.
..........*******./........... ..........|____|./...........
............................... ...............................
Index : 1234567890123456789012345678901 1234567890123456789012345678901
If you observe, there are 12 stars in the left figure, they are:
• 1 ascending slope (at index: 15),
• 3 descending slopes (at indexes: 11, 12, and 13),
• 3 flat lands (at indexes: 14, 16, and 17), and
• 5 full lands (2 at index 11, 1 at index 12, 1 at index 16, and another 1 at index 17).
Therefore, the cost of leveling [11, 17] is: 1 * 0.5 + 3 * 0.5 + 3 * 0 + 5 * 1 = 7.
In this example, [11, 17] is not the best choice, you can do better.
Input
The first line of input contains T (T ≤ 50) denoting the number of cases. Each case begins with two
integers N and M (1 ≤ M ≤ N ≤ 1, 000, 000) denoting the total length of the land and the length of the
land which should be leveled respectively. The following line contains a string of length N describing
the land’s surface. The string will only contain character ‘/’, ‘\’, or ‘ ’, as described in the problem
statement.
Output
For each case, output ‘Case #X: Y ’, where X is the case number starts from 1 and Y is the minimum
amount of land which should be disposed to achieve a level land which length is M for that particular
case. Output this number with exactly one digit after the decimal point.
Explanation for 1st sample case:
This is the same case as the example in the problem statement. The minimum amount of land
which you should dispose is 3.5. You can achieve this by leveling lands at [25, 31].
__ /_ __ __ /_ __
/.._/...\ /..\ _ /.._/...\ /..* *
Land : /..........\ ___ /....___/. /..........\ ___ /...*******
............_/.../........... ............_/.../...........
............................... ...............................
Index : 1234567890123456789012345678901 1234567890123456789012345678901
You will dispose: 1 ascending slope (at index 30), 2 descending slopes (at index 15 and 16), 4 flat
lands (at index 27, 28, 29, and 31), and 2 full lands (at index 15 and 31). Therefore the total cost will
be: 1 * 0.5 + 2 * 0.5 + 4 * 0 + 2 * 1 = 3.5.
Explanation for 2nd sample case:
If you level the land at [3, 6] or [4, 7], you don’t need to dispose any land as they are already level
(have the same height).
Explanation for 3rd sample case:
Level the land at [8, 11], and you only need to dispose 1 ascending slope and 1 descending slope.
Sample Input
4
31 7
//_//_\_////\/
10 4
//____\/
12 4
\\///_
12 1
//////\
Sample Output
Case #1: 3.5
Case #2: 0.0
Case #3: 1.0
Case #4: 0.5
Hint
题意
给你一个类似山峰的东西，你可以使得一个连续的m长度的山峰变成这一块的最低值。
然后问你最小的花费是多少。
（题意还是比较烦的，自己读读吧，我说不是很清楚……
题解：
考虑滑块，我们维护区间和，和区间最小值，那么花费就是区间和减去区间最小值乘以这个区间的大小就好了。
然后我们类似滑块去维护就好了。
O（n）就用单调队列去维护最小值，前缀和维护区间和就行了。
nlogn的做法就相当多了……
代码
#include<bits/stdc++.h>
#define two(x) (1<<(x))
using namespace std;
const int maxn = 1e6+7;
int a[maxn],b[maxn];
char s[maxn];
int mm[maxn];
int c[maxn][21];
int two[maxn];
void initrmp(int n)
{
mm[0]=-1;
for(int i=1;i<=n;i++){
mm[i]=((i&(i-1))==0)?mm[i-1]+1:mm[i-1];
}
}
int query(int l,int r){
int k = mm[r-l+1];
return min(c[l][k],c[r-(1<<k)+1][k]);
}
int cas = 0;
void solve(){
int n,m;
scanf("%d%d",&n,&m);
scanf("%s",s+1);
initrmp(n);
int now = 0;
for(int i=1;i<=n;i++){
if(s[i]=='/')a[i]=now,b[i]=1,now++;
if(s[i]=='\\')now--,a[i]=now,b[i]=1;
if(s[i]=='_')a[i]=now,b[i]=0;
c[i][0]=a[i];
}
for(int j=1;j<21;j++) for(int i = 1 ; i + ( 1 << j ) - 1 <= n ; ++ i) c[i][j]=min( c[i][j-1] , c[i + two(j-1)][j-1] );
long long sum = 0;
long long sum2 = 0;
for(int i=1;i<=m;i++){
sum+=1LL*a[i];
sum2+=1LL*b[i];
}
double Ans = 1e9;
Ans = 1.0*sum+0.5*sum2-1.0*m*query(1,m);
for(int i=m+1;i<=n;i++){
sum+=1LL*a[i]-1LL*a[i-m];
sum2+=1LL*b[i]-1LL*b[i-m];
Ans=min(Ans,1.0*sum+0.5*sum2-1.0*m*query(i-m+1,i));
}
printf("Case #%d: %.1f\n",++cas,Ans);
}
int main(){
//freopen("1.txt","r",stdin);
int t;
scanf("%d",&t);
while(t--)solve();
return 0;
}