You are given two integers n and m. Calculate the number of pairs of arrays (a,b) such that:
the length of both arrays is equal to m;
each element of each array is an integer between 1 and n (inclusive);
ai≤bi for any index i from 1 to m;
array a is sorted in non-descending order;
array b is sorted in non-ascending order.
As the result can be very large, you should print it modulo 109+7.
Input
The only line contains two integers n and m (1≤n≤1000, 1≤m≤10).
Output
Print one integer – the number of arrays a and b satisfying the conditions described above modulo 109+7.
input
10 1
output
55
有給一個n一個m,然後給兩個數組a,b。求滿足
1,數組的長度都是m
2,數組內的元素都不超過n
3,a數組遞增,b數組遞減
4,a[i]<b[i],即a[m]<b[m]
通過觀察,a數組是遞增的,b數組是遞減的,b數組逆置過來就是遞增的,而且b[m]>a[m],逆置過來之後b[m]就變成了b[1],b[1]大於等於a[m],所以相連之後就變成了一個長度爲2m,整體不遞減的序列,所以我們就求一個長度爲2m,數的範圍在1-n之內的所有符合條件的序列。
這裏我們定義dp[ i ][ j ]爲長度爲i,最後一個數是j的情況下的最優解,所以我們可以得到狀態轉移方程 dp[ i ][ j ]=dp[ i ][ j ]+dp[i-1][1-j];就是說長度爲i,最後一個數是j時候的解是由長度爲i-1,最後一個數小於j的所有狀態轉移過來的。最後在把長度爲2m的所有情況加上即使答案。
AC代碼:
#include<bits/stdc++.h>
#include<bitset>
#include<unordered_map>
#define pb push_back
#define bp __builtin_popcount
using namespace std;
typedef long long ll;
const int inf=0x3f3f3f3f;
const int maxn=1e3+100;
const int MOD=1e9+7;
int lowbit(int x){return x&-x;}
inline ll dpow(ll a, ll b){ ll r = 1, t = a; while (b){ if (b & 1)r = (r*t) % MOD; b >>= 1; t = (t*t) % MOD; }return r; }
inline ll fpow(ll a, ll b){ ll r = 1, t = a; while (b){ if (b & 1)r = (r*t); b >>= 1; t = (t*t); }return r; }
ll dp[maxn][maxn];//dp[i][j]長度爲i,以j結尾的最優解
int main()
{
ios::sync_with_stdio(false);
int n,m;
cin>>n>>m;
for(int i=1;i<=n;i++)
dp[1][i]=1;
for(int i=2;i<=2*m;i++)
{
for(int j=1;j<=n;j++)
{
for(int k=1;k<=j;k++)
{
dp[i][j]=(dp[i][j]+dp[i-1][k])%MOD;
}
}
}
ll ans=0;
for(int i=1;i<=n;i++)
{
ans=(ans+dp[2*m][i])%MOD;
}
cout<<ans<<endl;
//system("pause");
return 0;
}