【LOJ】#2350. 「JOI 2017/2018 决赛」月票购买

题解

首先求一个最短路图出来,最短路图就是这条边在最短路上就保留,否则就不保留,注意最短路图是一个有向图,一条边被保留的条件是
dis(S,u) + val(u,v) = dis(v,T)我们需要求两遍最短路

然后我们发现就相当于在最短路图上走一段,然后走一段非0的部分
我们把旧图保留,在上面连一些边权为0的有向边,从U到V求一遍最短路,由于最短路图是有向的,我们再从V到U求一遍最短路

然而……WA了?
我们发现我们最短路图上下来之后,可能会又走到最短路图上,但是同时选中两条路径是不可能的,我们希望的是在最短路上走一段,下来,并且再也不回到最短路图上

这样我们考虑分层图,设置三个状态,第一个状态是没有走到最短路图上,第二个状态是走到了最短路图上,第三个状态是走下来

所以要写两个dij,感觉代码还是十分的不优美,但是思路比较巧妙?

代码

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
//#define ivorysi
#define pb push_back
#define space putchar(' ')
#define enter putchar('\n')
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define mo 974711
#define MAXN 100005
#define MAXM 200005
#define eps 1e-3
#define RG register
#define calc(x) __builtin_popcount(x)
#define pLI pair
using namespace std;
typedef long long int64;
typedef double db;
template
void read(T &res) {
    res = 0;char c = getchar();T f = 1;
    while(c < '0' || c > '9') {
    if(c == '-') f = -1;
    c = getchar();
    }
    while(c >= '0' && c <= '9') {
    res = res * 10 + c - '0';
    c = getchar();
    }
    res *= f;
}
template
void out(T x) {
    if(x < 0) {putchar('-');x = -x;}
    if(x >= 10) {
    out(x / 10);
    }
    putchar('0' + x % 10);
}
int N,M,S,T,U,V;
struct Edge {
    int u,v;int64 c;
}C[MAXM];
struct Edge_link {
    struct node {
    int to,next;int64 val;
    }E[MAXM * 4];
    int head[MAXN],sumE;
    void add(int u,int v,int64 c) {
    E[++sumE].to = v;
    E[sumE].next = head[u];
    E[sumE].val = c;
    head[u] = sumE;
    }
}MK[2];
priority_queue Q;
int64 dis[3][MAXN];
bool vis[MAXN * 3];
void dijkstra(int64 *D,int id,int s) {
    for(int i = 1 ; i <= N ; ++i) D[i] = 1e18;
    memset(vis,0,sizeof(vis));
    D[s] = 0;
    while(!Q.empty()) Q.pop();
    Q.push(mp(0,s));
    while(!Q.empty()) {
    pLI now = Q.top();Q.pop();
    now.fi = -now.fi;
    if(vis[now.se]) continue;
    vis[now.se] = 1;
    D[now.se] = now.fi;
    for(int i = MK[id].head[now.se] ; i ; i = MK[id].E[i].next) {
        int v = MK[id].E[i].to;
        if(!vis[v] && D[now.se] + MK[id].E[i].val < D[v]) {
        D[v] = D[now.se] + MK[id].E[i].val;
        Q.push(mp(-D[v],v));
        }
    }
    }
}
void Another(int id,int s) {
    for(int i = 1 ; i <= N ; ++i) {
    for(int j = 0 ; j < 3 ; ++j) dis[j][i] = 1e18;
    }
    dis[0][s] = 0;dis[1][s] = 0;dis[2][s] = 0;
    memset(vis,0,sizeof(vis));
    while(!Q.empty()) Q.pop();
    Q.push(mp(0,s)),Q.push(mp(0,N + s)),Q.push(mp(0,2 * N + s));
    while(!Q.empty()) {
    pLI now = Q.top();Q.pop();
    now.fi = -now.fi;
    if(vis[now.se]) continue;
    int t = (now.se - 1) / N,u = now.se - t * N;
    dis[t][u] = now.fi;
    vis[now.se] = 1;
    for(int i = MK[id].head[u] ; i ; i = MK[id].E[i].next) {
        int v = MK[id].E[i].to;
        if(MK[id].E[i].val == 0 && t != 1) continue;
        if(t == 2) {
        if(MK[id].E[i].val != 0) {
            if(dis[2][v] > dis[2][u] + MK[id].E[i].val) {
            dis[2][v] = dis[2][u] + MK[id].E[i].val;
            Q.push(mp(-dis[2][v],2 * N + v));
            }
        }
        }
        if(t == 1) {
        if(MK[id].E[i].val == 0 && dis[1][v] > dis[1][u]) {
            dis[1][v] = dis[1][u];
            Q.push(mp(-dis[1][v],N + v));
        }
        else if(dis[2][v] > dis[1][u] + MK[id].E[i].val){
            dis[2][v] = dis[1][u] + MK[id].E[i].val;
            Q.push(mp(-dis[2][v],2 * N + v));
        }
        }
        if(t == 0) {
        if(MK[id].E[i].val != 0) {
            if(dis[0][v] > dis[0][u] + MK[id].E[i].val) {
            dis[0][v] = dis[0][u] + MK[id].E[i].val;
            Q.push(mp(-dis[0][v],v));
            }
            if(dis[1][v] > dis[0][u] + MK[id].E[i].val) {
            dis[1][v] = dis[0][u] + MK[id].E[i].val;
            Q.push(mp(-dis[1][v],v + N));
            }
        }
        }
    }
    }
}
void Solve() {
    read(N);read(M);
    read(S);read(T);read(U);read(V);
    int u,v;int64 c;
    for(int i = 1 ; i <= M ; ++i) {
    read(u);read(v);read(c);
    C[i] = (Edge){u,v,c};
    MK[0].add(u,v,c);MK[0].add(v,u,c);
    }
    dijkstra(dis[0],0,S);
    dijkstra(dis[1],0,T);
    for(int i = 1 ; i <= M ; ++i) {
    if(dis[0][C[i].u] + C[i].c + dis[1][C[i].v] == dis[0][T]) MK[1].add(C[i].u,C[i].v,0);
    else if(dis[0][C[i].v] + C[i].c + dis[1][C[i].u] == dis[0][T]) MK[1].add(C[i].v,C[i].u,0);
    MK[1].add(C[i].u,C[i].v,C[i].c),MK[1].add(C[i].v,C[i].u,C[i].c);
    }
    Another(1,U);
    int64 ans = min(min(dis[0][V],dis[1][V]),dis[2][V]);
    Another(1,V);
    ans = min(ans,min(dis[0][U],dis[1][U]));
    ans = min(ans,dis[2][U]);
    out(ans);enter;
}
int main() {
#ifdef ivorysi
    freopen("f1.in","r",stdin);
#endif
    Solve();
    return 0;
}

转载于:https://www.cnblogs.com/ivorysi/p/9146758.html

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