洛谷P4926 [1007]倍杀测量者(差分约束)

#include<bits/stdc++.h> 
#define MP(x, y) make_pair(x, y)
#define fi first
#define se second
#define LL long long
template <typename A, typename B> inline bool chmin(A &a, B b){if(a > b) {a = b; return 1;} return 0;}
template <typename A, typename B> inline bool chmax(A &a, B b){if(a < b) {a = b; return 1;} return 0;}
using namespace std;
const int MAXN = 4001, INF = 1e9;
const double eps = 1e-5;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
int N, M, K;
struct Edge {
    int v, op;
    double k, w; 
};
vector<Edge> v[MAXN];
void AddEdge(int x, int y, double w, int opt, double k) {
    v[x].push_back({y, opt, k, w});
}
double dis[MAXN];
bool vis[MAXN];
int times[MAXN];
bool SPFA(double add) {
    queue<int> q; q.push(N + 1);
    for(int i = 0; i <= N; i++) dis[i] = -1e18, vis[i] = times[i] = 0;
    dis[N + 1] = 0; ++times[N + 1];
    while(!q.empty()) {
        int p = q.front(); q.pop(); vis[p] = 0;
        for(auto &x : v[p]) {
            int opt = x.op, to = x.v; double k = x.k, w;
            if(opt == 0) w = x.w;
            else if(opt == 1) w = log2(k - add);
            else w = -log2(k + add);
            if(dis[to] < dis[p] + w) {
                dis[to] = dis[p] + w;
                if(!vis[to]) {
                    q.push(to);
                    vis[to] = 1;
                    ++times[to];
                    if(times[to] >= N + 1) return 0; 
                }
            }
        }
    }
    return 1;
}
signed main() {
    N = read(); M = read(); K = read();
    double l = 0, r = 10;
    for(int i = 1; i <= M; i++) {
        int opt = read(), x = read(), y = read(); double k = read();
        AddEdge(y, x, 0, opt, k);
        if(opt == 1) chmin(r, k);
    }
    for(int i = 1; i <= K; i++) {
        int c = read(); double x = read();
        AddEdge(0, c, log2(x), 0, 0);
        AddEdge(c, 0, -log2(x), 0, 0);
    }
    for(int i = 0; i <= N; i++) AddEdge(N + 1, i, 0, 0, 0);
    if(SPFA(0))  return puts("-1"), 0;
    while(r - l > eps) {
        double mid = (r + l) / 2;
        if(SPFA(mid)) r = mid;
        else l = mid;
    }
    printf("%lf", l);
    return 0;
}