51nod 1799 二分答案

寫個類似的遞歸過程，以k爲目標數。

計算階乘的時候，打表水過。。。

//#include<bits/stdc++.h>
#include<stdio.h>
#include<cstring>
#include<cmath>
#include<iostream>
using namespace std;

const long long mod=1e9+7;

long long a[110]={1,682498929,491101308,76479948,723816384,67347853,27368307,
625544428,199888908,888050723,927880474,281863274,661224977,623534362,
970055531,261384175,195888993,66404266,547665832,109838563,933245637,
724691727,368925948,268838846,136026497,112390913,135498044,217544623,
419363534,500780548,668123525,128487469,30977140,522049725,309058615,
386027524,189239124,148528617,940567523,917084264,429277690,996164327,
358655417,568392357,780072518,462639908,275105629,909210595,99199382,
703397904,733333339,97830135,608823837,256141983,141827977,696628828,
637939935,811575797,848924691,131772368,724464507,272814771,326159309,
456152084,903466878,92255682,769795511,373745190,606241871,825871994,
957939114,435887178,852304035,663307737,375297772,217598709,624148346,
671734977,624500515,748510389,203191898,423951674,629786193,672850561,
814362881,823845496,116667533,256473217,627655552,245795606,586445753,
172114298,193781724,778983779,83868974,315103615,965785236,492741665,
377329025,847549272,698611116};

int main()
{
	long long n,m,k,ans,l,r,mid,smal,bigg,last,i;
	while(~scanf("%lld%lld%lld",&n,&m,&k))
	{
		ans=1;
		l=1;r=n;
		smal=m;
		bigg=n-m;
		while(l<=r)
		{
			mid=(l+r)/2;
			if(mid<=k)
			{
				ans=(ans*smal)%mod;
				smal--;
				l=mid+1;
			}
			else
			{
				ans=(ans*bigg)%mod;
				bigg--;
				r=mid-1;
			}
		}
		last=bigg+smal;
		ans=(ans*a[last/10000000])%mod;
		for(i=last/10000000*10000000+1;i<=last;i++)
			ans=(ans*i)%mod;
		printf("%lld\n",ans);
	}
}