上圖來源於標程解析 CF

  1 #include<iostream>
  2 #include<algorithm>
  3 #include<cstdio>
  4 #include<vector>
  5 #define ll long long
  6 #define fir first
  7 #define sec second
  8 using namespace std;
  9 typedef pair<int, int> pii;
 10 const int N = 2e3 + 10;
 11 const
 
 int mod = 1e9 + 7;
 12 ll n, m, r;
 13 ll b[N];
 14 
 15 ll sum[N][N];//與原點包圍的面積內蛇的個數 
 16 
 17 ll max(ll a, ll b)
 18 {
 19     if(a > b)    return a;
 20     return b;
 21 }
 22 
 23 ll min(ll a, ll b)
 24 {
 25     if(a > b)    return b;
 26     return a;
 27 }
 28 
 29 ll quick_pow(ll base, ll k)
 
 30 {
 31     ll res = 1;
 32     while(k)
 33     {
 34         if(k & 1){
 35             res *= base % mod;
 36             res %= mod;
 37         }
 38         base *= base;
 39         base %= mod;
 40         k >>= 1;
 41     }
 42     return res % mod;
 43 }
 44 
 45 ll inv[N << 1
 
];
 46 ll f[N << 1];
 47 
 48 void pre()//預處理
 49 {
 50     f[0] = inv[0] = 1;
 51     for(ll i = 1 ; i < N * 2 ; i++){
 52         f[i] = 1ll * f[i - 1] * i % mod;//階乘 
 53         inv[i] = quick_pow(i, mod - 2) % mod * inv[i - 1] % mod;//求階乘的逆元 
 54     }
 55 }
 56 
 57 ll getsum2(int x1, int y1, int x2, int y2)
 58 {
 59     x1 = max(1, x1);
 60     y1 = max(1, y1);
 61     x2 = min(1000, x2);
 62     y2 = min(1000, y2);
 63     if(x1 > x2 || y2 < y1)    return 0;
 64     ll tmp = sum[x2][y2] - sum[x1 - 1][y2] - sum[x2][y1 - 1] + sum[x1 - 1][y1 - 1];
 65     return tmp;
 66 }
 67 
 68 ll getsum1(int x1, int y1, int r)
 69 {
 70     return getsum2(x1 - r, y1 - r, x1 + r, y1 + r);
 71 }
 72 
 73 ll comb(ll n, ll m)
 74 {
 75     if(m > n || m < 0)    return 0;
 76     return f[n] % mod * inv[n - m] % mod * inv[m] % mod;
 77 }
 78 
 79 int main(){
 80     pre();
 81     scanf("%lld%lld%lld",&n,&m,&r);
 82     vector<pii> snakes;
 83     
 84     for(int i = 0 ; i < n ; i++){
 85         int x, y;
 86         scanf("%d%d%lld",&x,&y,&b[i]);
 87         sum[x][y]++;
 88         snakes.push_back(make_pair(x, y));
 89     }
 90     
 91     for(int i = 1 ; i < N ; i++){
 92         for(int j = 1 ; j < N ; j++){
 93             sum[i][j] += sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1];//容斥 
 94         }
 95     }
 96     
 97     ll res = 0;
 98     int sz = (int)snakes.size();
 99     for(int i = 0 ; i < sz ; i++){
100         for(int j = i ; j < sz ; j++){//        
101             int x1 = snakes[i].fir;
102             int y1 = snakes[i].sec;
103             int x2 = snakes[j].fir;
104             int y2 = snakes[j].sec;
105             
106             int X1 = max(x1 - r, x2 - r);
107             int Y1 = max(y1 - r, y2 - r);
108             int X2 = min(x1 + r, x2 + r);
109             int Y2 = min(y1 + r, y2 + r);
110             
111             ll w = getsum2(X1, Y1, X2, Y2);//i死j死
112             ll u = getsum1(x1, y1, r) - w;//i死j不死
113             ll v = getsum1(x2, y2, r) - w;//i不死j死 
114             
115             ll tmp = 0;
116             tmp += comb(n, m) - comb(n - w, m);//
117             if(tmp < 0)    tmp += mod;
118             
119             tmp += comb(n - w, m) - comb(n - u - w, m) - comb(n - v - w, m) + comb(n - u - v - w, m);//
120             tmp %= mod;            
121             if(tmp < 0){
122                 tmp += mod;
123             }
124 
125             if(i == j){
126                 res += tmp * b[i] % mod * b[j] % mod;
127             }else{
128                 res += (2ll * tmp % mod * b[i] % mod * b[j] % mod) % mod;
129             }
130             res %= mod;
131         }
132     }
133     printf("%lld\n",res % mod);
134     
135     return 0;
136 }

最後的選取部分還沒理解透徹

1.在w範圍內出現的情況

2.不在w內出現，且兩點均在u，v範圍內出現的情況