Memory and his friend Lexa are competing to get higher score in one popular computer game. Memory starts with score a and Lexa starts with score b. In a single turn, both Memory and Lexa get some integer in the range [ - k;k] (i.e. one integer among - k, - k + 1, - k + 2, ..., - 2, - 1, 0, 1, 2, ..., k - 1, k) and add them to their current scores. The game has exactly t turns. Memory and Lexa, however, are not good at this game, so they both always get a random integer at their turn.
Memory wonders how many possible games exist such that he ends with a strictly higher score than Lexa. Two games are considered to be different if in at least one turn at least one player gets different score. There are (2k + 1)2t games in total. Since the answer can be very large, you should print it modulo 109 + 7. Please solve this problem for Memory.
The first and only line of input contains the four integers a, b, k, and t (1 ≤ a, b ≤ 100, 1 ≤ k ≤ 1000, 1 ≤ t ≤ 100) — the amount Memory and Lexa start with, the number k, and the number of turns respectively.
Print the number of possible games satisfying the conditions modulo 1 000 000 007 (109 + 7) in one line.
1 2 2 1
6
1 1 1 2
31
2 12 3 1
0
In the first sample test, Memory starts with 1 and Lexa starts with 2. If Lexa picks - 2, Memory can pick 0, 1, or 2 to win. If Lexa picks - 1, Memory can pick 1 or 2 to win. If Lexa picks 0, Memory can pick 2 to win. If Lexa picks 1 or 2, Memory cannot win. Thus, there are3 + 2 + 1 = 6 possible games in which Memory wins.
分析:dp[i][j]表示第i轮获得分数为j的方案数;
计数时维护前缀和即可;
代码:
#include <iostream> #include <cstdio> #include <cstdlib> #include <cmath> #include <algorithm> #include <climits> #include <cstring> #include <string> #include <set> #include <map> #include <queue> #include <stack> #include <vector> #include <list> #define rep(i,m,n) for(i=m;i<=n;i++) #define rsp(it,s) for(set<int>::iterator it=s.begin();it!=s.end();it++) #define mod 1000000007 #define inf 0x3f3f3f3f #define vi vector<int> #define pb push_back #define mp make_pair #define fi first #define se second #define ll long long #define pi acos(-1.0) #define pii pair<int,int> #define Lson L, mid, rt<<1 #define Rson mid+1, R, rt<<1|1 const int maxn=5e5+10; using namespace std; ll gcd(ll p,ll q){return q==0?p:gcd(q,p%q);} ll qpow(ll p,ll q){ll f=1;while(q){if(q&1)f=f*p;p=p*p;q>>=1;}return f;} int n,m,k,t; ll dp[2][maxn],sum[maxn],a,b; int main() { int i,j; scanf("%lld%lld%d%d",&a,&b,&k,&t); dp[0][0]=1; int now=1; rep(i,0,2*k*t)sum[i]=1; rep(i,1,t) { rep(j,0,2*k*t) { if(j<=2*k)dp[now][j]=sum[j]; else dp[now][j]=(sum[j]-sum[j-2*k-1]+mod)%mod; } sum[0]=dp[now][0]; rep(j,1,2*k*t+100) { sum[j]=(sum[j-1]+dp[now][j])%mod; } now^=1; } ll ans=0; rep(i,0,2*k*t) { if(i+a-b-1>=0)ans=(ans+dp[now^1][i]*sum[i+a-b-1]%mod)%mod; } printf("%lld ",ans); //system("Pause"); return 0; }