bzoj 5206

$n$ 点 $m$ 边图的有限制三元环个数

首先将所有左右端点并且属性相同的边的权值相加，合并为一条边

在这只之前得先排序
排序之前得先判断是否需要交换左右端点的位置 T_T

然后统计三元环

补充说明按照上一篇博客的做法统计的正确性

考虑一个三元环 $(u, v), (v, v_2), (u, v_2)$
建边之后一定存在且只存在一个点的出度为2，这也就是改图成为 $DAG$ 的原因

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>

using namespace std;

#define gc getchar()
inline int read() {int x = 0; char c = gc; while(c < '0' || c > '9') c = gc; 
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = gc; return x;}
#undef gc

const int N = 5e4 + 10, M = 1e5 + 10, Mod = 1e9 + 7;

int n, m;
int A[M], B[M], W[M], C[M], Id[M];
int du[N];
int vis[N][4], cost[N][4];
 
inline int Get_() {
    char c = getchar();
    return c == 'R' ? 1 : (c == 'G' ? 2 : 3);
}

inline bool Cmp(const int &a, const int &b) {
    if(A[a] != A[b]) return A[a] < A[b];
    if(B[a] != B[b]) return B[a] < B[b];
    return C[a] < C[b];
}

int cnt, head[N];
struct Node {int v, w, nxt, col;} G[M];

inline void Add(int u, int v, int w, int col) {
    G[++ cnt].v = v;
    G[cnt].w = w;
    G[cnt].col = col;
    G[cnt].nxt = head[u];
    head[u] = cnt;
}

int main() {
    n = read(), m = read();
    for(int i = 1; i <= m; i ++) {
        A[i] = read(), B[i] = read(), W[i] = read(), C[i] = Get_(), Id[i] = i;
        if(A[i] > B[i]) swap(A[i], B[i]);    
    }
    sort(Id + 1, Id + m + 1, Cmp);
    int t = 1;
    for(int i = 2; i <= m; i ++) {
        if(A[Id[i]] == A[Id[t]] && B[Id[i]] == B[Id[t]] && C[Id[i]] == C[Id[t]]) W[Id[t]] = (W[Id[t]] + W[Id[i]]) % Mod;
        else t ++, Id[t] = Id[i];
    }
    m = t;
    for(int i = 1; i <= m; i ++) du[A[Id[i]]] ++, du[B[Id[i]]] ++;
    for(int i = 1; i <= n; i ++) head[i] = -1;
    for(int i = 1; i <= m; i ++) {
        if(du[A[Id[i]]] > du[B[Id[i]]] || (du[A[Id[i]]] == du[B[Id[i]]] && A[Id[i]] > B[Id[i]])) 
            Add(B[Id[i]], A[Id[i]], W[Id[i]], C[Id[i]]);
        else Add(A[Id[i]], B[Id[i]], W[Id[i]], C[Id[i]]);    
    }
    long long Answer(0);
    for(int k = 1; k <= m; k ++) {
        for(int i = head[A[Id[k]]]; ~ i; i = G[i].nxt)
            if(G[i].col != C[Id[k]]) vis[G[i].v][G[i].col] = k, cost[G[i].v][G[i].col] = G[i].w;
        long long tot(0);
        for(int i = head[B[Id[k]]]; ~ i; i = G[i].nxt) {
            if(G[i].col != C[Id[k]]) {
                int need_col = 6 - C[Id[k]] - G[i].col;
                if(vis[G[i].v][need_col] == k) tot = (tot + 1LL * cost[G[i].v][need_col] * G[i].w) % Mod;
            }
        }
        Answer = (Answer + (tot * W[Id[k]]) % Mod) % Mod;
    }
    cout << Answer << "
";
    return 0;
}