CodeForces 479E Riding in a Lift

 1 // File Name: 479e.cpp
 2 // Author: darkdream
 3 // Created Time: 2015年03月08日 星期日 21时37分17秒
 4 
 5 #include<vector>
 6 #include<list>
 7 #include<map>
 8 #include<set>
 9 #include<deque>
10 #include<stack>
11 #include<bitset>
12 #include<algorithm>
13 #include<functional>
14 #include<numeric>
15 #include<utility>
16 #include<sstream>
17 #include<iostream>
18 #include<iomanip>
19 #include<cstdio>
20 #include<cmath>
21 #include<cstdlib>
22 #include<cstring>
23 #include<ctime>
24 #define LL long long
25 
26 using namespace std;
27 #define maxn 5005
28 #define M 1000000007
29 LL dp[maxn];
30 LL tmp[maxn];
31 int main(){
32    int n,a,b,k; 
33    scanf("%d %d %d %d",&n,&a,&b,&k);
34    dp[a] = 1;
35    for(int i = 1;i <= k ;i ++)
36    {
37      memset(tmp,0,sizeof(tmp));
38      for(int j = 1;j <= n ;j ++)
39      {
40          if(j == b )
41              continue;
42          int dis = abs(j-b)-1;
43          int l = max(j-dis,1); 
44          int r = min(n,j+dis);
45          tmp[l] = (tmp[l]+dp[j])%M;
46          tmp[j] = (tmp[j]-dp[j])%M;
47          tmp[j+1] =(tmp[j+1]+dp[j])%M;
48          tmp[r+1] =(tmp[r+1]-dp[j])%M;
49      }
50      for(int j = 1;j <= n;j ++)
51      {
52         tmp[j] = (tmp[j-1] +M + tmp[j])%M;    
53      }
54      for(int j= 1;j <= n;j ++)
55          dp[j] = tmp[j];
56      dp[b] = 0;
57    }
58    LL sum =0 ; 
59    for(int i = 1;i <= n;i ++)
60    {
61       sum = (sum + dp[i]) % M;
62       //printf("%I64d ",dp[i]);
63    }
64    printf("%I64d\n",sum);
65 return 0;
66 }