1799 二分答案

 1 #include <cstdio>
 2 #include <iostream>
 3 #include <algorithm>
 4 #include <cstring>
 5 using namespace std;
 6 #define LL long long
 7 #define MOD 1000000007
 8 
 9 int n,m,k;
10 const int sb [] =
11 {1, 682498929, 491101308, 76479948, 723816384,
12 67347853, 27368307, 625544428, 199888908, 888050723, 927880474,
13 281863274, 661224977, 623534362, 970055531, 261384175, 195888993,
14 66404266, 547665832, 109838563, 933245637, 724691727, 368925948,
15  268838846, 136026497, 112390913, 135498044, 217544623, 419363534,
16   500780548, 668123525, 128487469, 30977140, 522049725, 309058615,
17   386027524, 189239124, 148528617, 940567523, 917084264, 429277690,
18   996164327, 358655417, 568392357, 780072518, 462639908, 275105629,
19   909210595, 99199382, 703397904, 733333339, 97830135, 608823837,
20   256141983, 141827977, 696628828, 637939935, 811575797, 848924691,
21   131772368, 724464507, 272814771, 326159309, 456152084, 903466878,
22   92255682, 769795511, 373745190, 606241871, 825871994, 957939114,
23   435887178, 852304035, 663307737, 375297772, 217598709, 624148346,
24   671734977, 624500515, 748510389, 203191898, 423951674, 629786193,
25   672850561, 814362881, 823845496, 116667533, 256473217, 627655552,
26   245795606, 586445753, 172114298, 193781724, 778983779, 83868974,
27   315103615, 965785236, 492741665, 377329025, 847549272, 698611116};
28 
29 LL get(LL x)
30 {
31     LL ret = sb[x/(int)1e7];
32     int l = x/((int)1e7);
33     for (int i=l*((int)1e7)+1;i<=x;i++)
34         ret=ret*i%MOD;
35     return ret;
36 }
37 
38 int main()
39 {
40     scanf("%d%d%d",&n,&m,&k);
41     int x =0 ,y = 0;
42     int l=1,r=n;
43     while (l<=r)
44     {
45         int mid = (l+r)>>1;
46         if (mid<=k) l=mid+1,x++;
47         else r=mid-1,y++;
48     }
49     LL ans = 1;
50     for (int i=m;i>=m-x+1;--i) ans=ans*i%MOD;
51     for (int i=n-m;i>=n-m-y+1;--i) ans=ans*i%MOD;
52 
53     printf("%lld\n",ans*get(n-x-y)%MOD);
54     return 0;
55 }