hdu 6201 transaction transaction transaction（最长路）

transaction transaction transaction

题目链接：transaction transaction transaction

题意：给你一棵树，每个点都有点权，每条边也有边权，要求选择起点S和终点T，使得val点T−val点S−val边最大

思路：建立源点和汇点，源点到树上所有点的权值为p[i]，树上所有点到汇点的权值为−p[i]，然后直接建图，spfa跑最长路即可

代码：

#include<bits/stdc++.h>
using namespace std;

const int maxn=1e5+10;
struct edge
{
    int v,w,next;
} E[maxn<<2];
int inq[maxn],dis[maxn],first[maxn],p[maxn];
int n,len;

inline void scan_d(int &ret)
{
    char c;
    ret = 0;
    while ((c = getchar()) < '0' || c > '9');
    while (c >= '0' && c <= '9')
        ret = ret * 10 + (c - '0'), c = getchar();
}

int spfa(int st,int ed)
{
    memset(inq,0,sizeof(inq));
    memset(dis,0,sizeof(dis));
    queue<int>q;
    q.push(st),inq[st]=1;
    while(!q.empty())
    {
        st=q.front();
        q.pop(),inq[st]=0;
        for(int i=first[st]; ~i; i=E[i].next)
        {
            int v=E[i].v,w=E[i].w;
            if(dis[v]<dis[st]+w)
            {
                dis[v]=dis[st]+w;
                if(!inq[v])
                {
                    inq[v]=1;
                    q.push(v);
                }
            }
        }
    }
    return dis[ed];
}

void add_edge(int u,int v,int w)
{
    E[len].v=v,E[len].w=w,E[len].next=first[u],first[u]=len++;
}

int main()
{
    int t;
    scan_d(t);
    while(t--)
    {
        memset(first,-1,sizeof(first));
        scan_d(n);
        for(int i=1; i<=n; ++i)
            scan_d(p[i]);
        len=0;
        for(int i=1; i<=n; ++i)
            add_edge(0,i,p[i]),add_edge(i,n+1,-p[i]);
        int u,v,w;
        for(int i=1; i<n; ++i)
        {
            scan_d(u),scan_d(v),scan_d(w);
            add_edge(u,v,-w),add_edge(v,u,-w);
        }
        printf("%d\n",spfa(0,n+1));
    }
    return 0;
}

直接跑n边spfa竟然没超时

代码：

#include<stdio.h>
#include<queue>
#include<string.h>
#include<algorithm>
using namespace std;

const int maxn=1e5+10;
struct edge
{
    int v,w,next;
} E[maxn<<1];
int p[maxn],d[maxn],first[maxn];
int n,len;

inline void scan_d(int &ret)
{
    char c;
    ret = 0;
    while ((c = getchar()) < '0' || c > '9');
    while (c >= '0' && c <= '9')
        ret = ret * 10 + (c - '0'), c = getchar();
}

void spfa(int u)
{
    queue<int>q;
    q.push(u);
    while(!q.empty())
    {
        u=q.front();
        q.pop();
        for(int i=first[u]; ~i; i=E[i].next)
        {
            int v=E[i].v,w=E[i].w;
            if(d[v]<d[u]+w)
            {
                d[v]=d[u]+w;
                q.push(v);
            }
        }
    }
}

void add_edge(int u,int v,int w)
{
    E[len].v=v,E[len].w=w,E[len].next=first[u],first[u]=len++;
}

int main()
{
    int t;
    scan_d(t);
    while(t--)
    {
        scan_d(n);
        for(int i=1; i<=n; ++i)
            scan_d(p[i]);
        memset(first,-1,sizeof(first));
        len=0;
        int u,v,w;
        for(int i=1; i<n; ++i)
        {
            scan_d(u),scan_d(v),scan_d(w);
            add_edge(u,v,p[v]-p[u]-w);
            add_edge(v,u,p[u]-p[v]-w);
        }
        memset(d,0,sizeof(d));
        for(int i=1; i<=n; ++i)
            spfa(i);
        int maxx=0;
        for(int i=1; i<=n; ++i)
            maxx=max(maxx,d[i]);
        printf("%d\n",maxx);
    }
    return 0;
}