SRM467

250pt：

一个学生等老师来上课的，但是他不知道老师啥时候会来的，然后他等waiting时间后觉得无聊就会出去转walking时间，回来等待waiting时间后老师没来就会再次出去。老师会在a...b区间时间任意时刻来，是等概率的。但是老师等t时间后，就会不会让你进来了的。让你求你进不去教室的概率是多少。（所有的数<= 1000W）

思路:因为小于1000w，所以我就暴力把每一分钟的状态都统计出来。

注意长度为0的情况即可（比如老师即到即走的情况）举要特判

#line 7 "LateProfessor.cpp"
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <fstream>
#include <numeric>
#include <iomanip>
#include <bitset>
#include <list>
#include <stdexcept>
#include <functional>
#include <utility>
#include <ctime>
using namespace std;

#define PB push_back
#define MP make_pair

#define REP(i,n) for(i=0;i<(n);++i)
#define FOR(i,l,h) for(i=(l);i<=(h);++i)
#define FORD(i,h,l) for(i=(h);i>=(l);--i)

typedef vector<int> VI;
typedef vector<string> VS;
typedef vector<double> VD;
typedef long long LL;
typedef pair<int,int> PII;


class LateProfessor
{
        public:
        double getProbability(int wait, int walk, int late, int st, int end)
        {
              double sum = (end - st);
              int m = wait + walk;
              double ans = 0;
            //  int last = -1, len = 0;
              if (st == end){
                    if (st % m <= wait || (st % m > wait && st % m + late > m)) return 0.0;
                    return 1.0;       
              }
              for (int i = st; i < end; ++i){
                    if (i % m < wait || (i % m >= wait && i % m + late >= m)) ++ans;
              }
              return 1 - ans / sum;
        }

};

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500pt：

给定supersum定义如下

SuperSum(0 , n) = n, for all positive n.
SuperSum(k , n) = SuperSum(k-1 , 1) + SuperSum(k-1 , 2) + ... + SuperSum(k-1 , n), for all positive k, n.

给定k,n(k <= 50, n <= 10^9),求supersum（k,n）

思路：乍一看不会做，然后就打了个20*20的表，发现斜着看就是一个杨辉三角，然后直接算可以了

答案就是C(n + k, k + 1)

 1 #line 7 "SuperSum.cpp"
 2 #include <cstdlib>
 3 #include <cctype>
 4 #include <cstring>
 5 #include <cstdio>
 6 #include <cmath>
 7 #include <algorithm>
 8 #include <vector>
 9 #include <string>
10 #include <iostream>
11 #include <sstream>
12 #include <map>
13 #include <set>
14 #include <queue>
15 #include <stack>
16 #include <fstream>
17 #include <numeric>
18 #include <iomanip>
19 #include <bitset>
20 #include <list>
21 #include <stdexcept>
22 #include <functional>
23 #include <utility>
24 #include <ctime>
25 using namespace std;
26 
27 #define PB push_back
28 #define MP make_pair
29 
30 #define REP(i,n) for(i=0;i<(n);++i)
31 #define FOR(i,l,h) for(i=(l);i<=(h);++i)
32 #define FORD(i,h,l) for(i=(h);i>=(l);--i)
33 #define M 1000000007
34 typedef vector<int> VI;
35 typedef vector<string> VS;
36 typedef vector<double> VD;
37 typedef long long LL;
38 typedef pair<int,int> PII;
39 
40 class SuperSum
41 {
42         public:
43         long long power(long long a, long long b){
44              long long ret = 1;
45              while (b){
46                  if (b&1) ret = (ret * a) % M;
47                  b >>= 1;
48                  a = (a * a) % M;
49              }
50              return ret;
51         }
52         long long C(int n, int m){
53              long long ret = 1;
54              for (int i = 1; i <= m; ++i){
55                  ret = (ret * power(i, M - 2)) % M;
56                  ret = (ret * (n - i + 1)) % M;
57              }
58              return ret;
59         }
60         int calculate(int K, int N)
61         {
62                 int n = K + N;
63                 int m = K + 1;
64                 return C(n, m);
65         }
66 
67 };

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