LOJ6432 [PKUSC2018] 真实排名 【组合数】

题目分析：

做三个指针然后预处理阶乘就行。

题目代码：

 #include<bits/stdc++.h>
 using namespace std;

 const int maxn = ;

 const int mod = ;

 int n,k;
 struct node{
     int data,num;
 }a[maxn];

 int ans[maxn];

 int fac[maxn],inv[maxn];

 int fast_pow(int now,int pw){
     if(pw == )return now;
     int z = fast_pow(now,pw/);
     z = (1ll*z*z)%mod;
     if(pw & ) z = (1ll*z*now)%mod;
     return z;
 }

 void init(){
     fac[] = ;
     for(int i=;i<=n;i++) fac[i] = (1ll*fac[i-]*i) %mod;
     inv[n] = fast_pow(fac[n],mod-);
     for(int i=n;i>=;i--){
     inv[i-] = (1ll*inv[i]*i)%mod;
     }
 }

 int C(int alpha,int beta){
     if(beta > alpha) return ;
     return (((1ll*fac[alpha]*inv[beta])%mod)*(inv[alpha-beta]))%mod;
 }

 int cmp(node alpha,node beta){return alpha.data > beta.data;}

 void read(){
     scanf("%d%d",&n,&k);
     for(int i=;i<=n;i++) {
     scanf("%d",&a[i].data),a[i].num = i;
     }
     sort(a+,a+n+,cmp);
 }

 void work(){
     init();
     int pts = ,ok = ;
     while(pts <= n && a[pts].data* >= a[].data) pts++;
     for(int i=;i<=n;i++){
     int nxt = i; while(nxt+<=n && a[nxt+].data == a[i].data) nxt++;
     while(pts <= n && a[pts].data* >= a[i].data) pts++;
     int forw = n-(pts-nxt),res = C(forw,k);
     for(int j=i;j<=nxt;j++) ans[a[j].num] += res;
     while(ok <= nxt && a[ok].data >= a[nxt].data*) ok++;
     int newm = k-(nxt-ok+);
     if(newm >= ){
         forw = n-(nxt-ok+),res = C(forw,newm);
         for(int j=i;j<=nxt;j++) ans[a[j].num] += res,ans[a[j].num] %= mod;
     }
     i = nxt;
     }
     for(int i=;i<=n;i++) if(a[i].data == ) ans[a[i].num] = C(n,k);
     for(int i=;i<=n;i++) printf("%d\n",ans[i]);
 }

 int main(){
     read();
     work();
     return ;
 }