Ant Counting POJ

Bessie was poking around the ant hill one day watching the ants march to and fro while gathering food. She realized that many of the ants were siblings, indistinguishable from one another. She also realized the sometimes only one ant would go for food, sometimes a few, and sometimes all of them. This made for a large number of different sets of ants!

Being a bit mathematical, Bessie started wondering. Bessie noted that the hive has T (1 <= T <= 1,000) families of ants which she labeled 1..T (A ants altogether). Each family had some number Ni (1 <= Ni <= 100) of ants.

How many groups of sizes S, S+1, ..., B (1 <= S <= B <= A) can be formed?

While observing one group, the set of three ant families was seen as {1, 1, 2, 2, 3}, though rarely in that order. The possible sets of marching ants were:

3 sets with 1 ant: {1} {2} {3}
5 sets with 2 ants: {1,1} {1,2} {1,3} {2,2} {2,3}
5 sets with 3 ants: {1,1,2} {1,1,3} {1,2,2} {1,2,3} {2,2,3}
3 sets with 4 ants: {1,2,2,3} {1,1,2,2} {1,1,2,3}
1 set with 5 ants: {1,1,2,2,3}

Your job is to count the number of possible sets of ants given the data above.
Input

* Line 1: 4 space-separated integers: T, A, S, and B

* Lines 2..A+1: Each line contains a single integer that is an ant type present in the hive

Output

* Line 1: The number of sets of size S..B (inclusive) that can be created. A set like {1,2} is the same as the set {2,1} and should not be double-counted. Print only the LAST SIX DIGITS of this number, with no leading zeroes or spaces.

Sample Input

3 5 2 3
1
2
2
1
3

Sample Output

10

Hint

INPUT DETAILS:

Three types of ants (1..3); 5 ants altogether. How many sets of size 2 or size 3 can be made?


OUTPUT DETAILS:

5 sets of ants with two members; 5 more sets of ants with three members

 

这个递推式没推出来，后天一定补出来。

不过套模板的时候，注意复杂度，该算法是O(nm)的，此题n<=1000,m<=1000*100,所以不论是空间还是时间上应该都必须在优化一下。不过优化的写法正在学习

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;

const int mod=1e6;
const int maxn=1005;
const int maxx=100005;

int T,A,S,B;
int no[maxn],dp[maxn][maxx];

void solve()
{   for(int i=0;i<=T;i++) dp[i][0]=1;
    for(int i=0;i<T;i++){
        for(int j=1;j<=B;j++){
            if(j-1-no[i]>=0) dp[i+1][j]=(dp[i+1][j-1]+dp[i][j]-dp[i][j-1-no[i]]+mod)%mod;
            else dp[i+1][j]=(dp[i+1][j-1]+dp[i][j])%mod;
        }
    }
    
    int ans=0;
    for(int i=S;i<=B;i++) ans=(ans+dp[T][i])%mod;
    cout<<ans<<endl;
}

int main()
{   cin>>T>>A>>S>>B;
    for(int i=1;i<=A;i++){    
        int x;scanf("%d",&x);
        no[x-1]++;
    }
    
    solve();
}