2018 Multi-University Training Contest 4 Problem B. Harvest of Apples 【莫队+排列组合+逆元预处理技巧】...-优快云博客

任意门：http://acm.hdu.edu.cn/showproblem.php?pid=6333

Problem B. Harvest of Apples

Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 262144/262144 K (Java/Others)
Total Submission(s): 4043 Accepted Submission(s): 1560

Problem Description

There are

Input

The first line of the input contains an integer

Output

For each test case, print an integer representing the number of ways modulo

Sample Input

5 2

1000 500

Sample Output

924129523

Source

2018 Multi-University Training Contest 4

题意概括：

有 N 个苹果，问最多选 m 个苹果的方案有多少种？

解题思路：

大佬讲的很好了。

https://blog.youkuaiyun.com/codeswarrior/article/details/81359075

推出四个公式，算是一道比较裸的莫队了。

Sm−1n=Smn−CmnSnm−1=Snm−Cnm
Sm+1n=Smn+Cm+1nSnm+1=Snm+Cnm+1(或者Smn=Sm−1n+CmnSnm=Snm−1+Cnm)

Smn+1=2Smn−CmnSn+1m=2Snm−Cnm
Smn−1=Smn+Cmn−12Sn−1m=Snm+Cn−1m2(或者Smn=Smn+1+Cmn2Snm=Sn+1m+Cnm2)

需要注意的点较多：

1、精度问题，注意数据范围

2、为了保证精度，除法需要转换为逆元，预处理逆元的技巧

AC code:

  1 #include<cstdio>
  2 #include<algorithm>
  3 #include<iostream>
  4 #include<cstring>
  5 #include<vector>
  6 #include<queue>
  7 #include<cmath>
  8 #include<set>
  9 #define INF 0x3f3f3f3f
 10 #define LL long long
 11 using namespace std;
 12 const LL MOD = 1e9+7;
 13 const int MAXN = 1e5+10;
 14 LL N, M;
 15 LL fac[MAXN], inv[MAXN];
 16 struct Query
 17 {
 18     int L, R, id, block;
 19     bool operator < (const Query &p)const{
 20         if(block == p.block) return R < p.R;
 21         return block < p.block;
 22     }
 23 }Q[MAXN];
 24 LL res, rev2;
 25 LL ans[MAXN];
 26 
 27 LL q_pow(LL a, LL b)
 28 {
 29     LL pans = 1LL;
 30     while(b){
 31         if(b&1) pans = pans*a%MOD;
 32         b>>=1LL;
 33         a = a*a%MOD;
 34     }
 35     return pans;
 36 }
 37 
 38 LL C(int n, int k)
 39 {
 40     return fac[n]*inv[k]%MOD*inv[n-k]%MOD;
 41 }
 42 
 43 void init()
 44 {
 45     rev2 = q_pow(2, MOD-2);                     // 2的逆元
 46     fac[0] = fac[1] = 1;
 47     for(LL i = 2; i < MAXN; i++){              //预处理阶乘
 48         fac[i] = fac[i-1]*i%MOD;
 49     }
 50 
 51     inv[MAXN-1] = q_pow(fac[MAXN-1], MOD-2);    //逆推预处理阶乘的逆元
 52     for(int i = MAXN-2; i >= 0; i--){
 53         inv[i] = inv[i+1]*(i+1)%MOD;
 54     }
 55 }
 56 
 57 void  addN(int posL, int posR)
 58 {
 59     res = (2*res%MOD-C(posL-1, posR)%MOD + MOD)%MOD;
 60 }
 61 
 62 void addM(int posL, int posR)
 63 {
 64     res = (res+C(posL, posR))%MOD;
 65 }
 66 
 67 void delN(int posL, int posR)
 68 {
 69     res = (res+C(posL-1, posR))%MOD*rev2%MOD;
 70 }
 71 
 72 void delM(int posL, int posR)
 73 {
 74     res = (res - C(posL, posR) + MOD)%MOD;
 75 }
 76 
 77 int main()
 78 {
 79     int T_case;
 80     init();
 81     int len = (int)sqrt(MAXN*1.0);
 82     scanf("%d", &T_case);
 83     for(int i = 1; i <= T_case; i++){
 84         scanf("%d%d", &Q[i].L, &Q[i].R);
 85         Q[i].id = i;
 86         Q[i].block = Q[i].L/len;
 87     }
 88     sort(Q+1, Q+1+T_case);
 89     res = 2;
 90     int curL = 1, curR = 1;
 91     for(int i = 1; i <= T_case; i++){
 92         while(curL < Q[i].L) addN(++curL, curR);
 93         while(curR < Q[i].R) addM(curL, ++curR);
 94         while(curL > Q[i].L) delN(curL--, curR);
 95         while(curR > Q[i].R) delM(curL, curR--);
 96         ans[Q[i].id] = res;
 97     }
 98     for(int i = 1; i <= T_case; i++){
 99         printf("%lld\n", ans[i]);
100     }
101 
102     return 0;
103 
104 }