Vijos 1082 丛林探险

题目链接：https://vijos.org/p/1082

本题本来是练习SPFA的。我一看DIscuss能用裸搜。果断敲了一个前向星+DFS，居然超时了。后来发现是Next数组开小了，应该开成两倍边数的大小。

后来我又把前向星改成邻接表，也AC了。。最后写一发SPFA。

前向星+DFS+剪枝：（AC）：

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

#define INF 0x3f3f3f3f
#define Maxn 5005
#define Maxm 40005<<1

struct Edge
{
    int x;
    int y;
    int c;
    int d;
};

Edge e[Maxm];
int first[Maxn];
int next[Maxm];//注意不要写成next[Maxn],切记
int used[Maxn];
int k;
int s,t;
int n,m;
int mma = INF;

void dfs(int start,int total_len,int total_k)
{
    //加两个剪支，体力值超过范围或者长度必然不是最优两种情况
    if(total_k > k || total_len > mma)
    {
        return;
    }
    if(start == t)
    {
        if(total_k<=k)
        {
            mma = mma>total_len ? total_len : mma;
        }
        return;
    }
    used[start] = 1;
    for(int i=first[start]; i!=-1; i=next[i])
    {
        if(used[e[i].y] == 0)
        {
            dfs(e[i].y,total_len + e[i].d ,total_k + e[i].c);
        }
    }
    used[start] = 0;
}
int main()
{
/*#ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
#endif*/
    scanf(" %d %d",&n,&m);
    memset(first,-1,sizeof(first));
    memset(next,-1,sizeof(next));
    memset(used,0,sizeof(used));

    int temp = m;
    int i = 0;
    while(temp--)
    {
        scanf(" %d %d %d %d",&e[i].x,&e[i].y,&e[i].c,&e[i].d);
        next[i] = first[e[i].x];
        first[e[i].x] = i;
        i++;
        e[i].x = e[i-1].y,e[i].y = e[i-1].x,e[i].c = e[i-1].c,e[i].d = e[i-1].d;
        next[i] = first[e[i].x];
        first[e[i].x] = i;
        i++;
    }
    scanf(" %d %d %d",&s,&t,&k);
    dfs(s,0,0);
    if(mma == INF) printf("-1\n");
    else printf("%d\n",mma);
}

邻接表+DFS+剪枝（AC）：

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

#define INF 0xffffff
#define Maxn 5005
#define Maxm 40005<<1

struct vertex
{
    int n;
    int length;
    int cost;
};

vector < vertex > graph[Maxn];
int used[Maxn];
int k;
int s,t;
int n,m;
int mma = INF;

void dfs(int start,int total_len,int total_k)
{
    //加两个剪支，体力值超过范围或者长度必然不是最优两种情况
    if(total_k <=k && total_len < mma)
    {
        if(start == t)
        {
            mma = total_len;
        }
        else
        {
            used[start] = 1;
            for(int i=0; i<graph[start].size(); i++)
            {
                if(used[graph[start][i].n] == 0)
                {
                    dfs(graph[start][i].n,total_len + graph[start][i].length,total_k + graph[start][i].cost);
                }
            }
            used[start] = 0;
        }
    }
}
int main()
{
    /*#ifndef ONLINE_JUDGE
        freopen("in.txt","r",stdin);
    #endif*/
    int st,vd,ct,le;
    scanf(" %d %d",&n,&m);
    memset(used,0,sizeof(used));
    int temp = m;
    int i = 0;
    while(temp--)
    {
        scanf(" %d %d %d %d",&st,&vd,&ct,&le);
        vertex one;
        one.n = vd;
        one.length = le;
        one.cost = ct;
        graph[st].push_back(one);

        vertex other;
        other.n = st;
        other.cost = ct;
        other.length = le;
        graph[vd].push_back(other);
    }
    scanf(" %d %d %d",&s,&t,&k);
    dfs(s,0,0);
    if(mma == INF) printf("-1\n");
    else printf("%d\n",mma);
}

前向星+SPFA (AC):

声明两个Dist数组，一个代表时间（距离），一个代表体力值，只有两个Dist同时满足条件时，才会更新，也才有机会入队列。

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

#define INF 0x3f3f3f3f
#define Maxn 5005
#define Maxm 40005<<1

struct Edge
{
    int x;
    int y;
    int c;
    int d;
};

Edge e[Maxm];
int first[Maxn];
int next[Maxm];
int used[Maxn];
int distC[Maxn];//体力
int distD[Maxn];//时间

int k;
int s,t;
int n,m;

void spfa(int s)
{
    queue<int> q;
    memset(used,0,sizeof(used));
    used[s] = 1;
    q.push(s);
    while(!q.empty())
    {
        int u = q.front();
        q.pop();
        used[u] = 0;
        for(int i=first[u]; i!=-1; i=next[i])
        {
            int v = e[i].y;
            int c = e[i].c;
            int d = e[i].d;
            if(distD[v] - d > distD[u] && distC[u] + c <=k)
            {
                distD[v] = distD[u] + d;
                distC[v] = distC[u] + c;
                if(used[v] == 0)
                {
                    used[v] = 1;
                    q.push(v);
                }
            }
        }
    }
}
int main()
{
    /*#ifndef ONLINE_JUDGE
        freopen("in.txt","r",stdin);
    #endif*/

    scanf(" %d %d",&n,&m);

    memset(first,-1,sizeof(first));
    memset(next,-1,sizeof(next));
    memset(used,0,sizeof(used));
    memset(distC,0x3f,sizeof(distC));
    memset(distD,0x3f,sizeof(distD));

    int temp = m;
    int i = 0;
    while(temp--)
    {
        scanf(" %d %d %d %d",&e[i].x,&e[i].y,&e[i].c,&e[i].d);
        next[i] = first[e[i].x];
        first[e[i].x] = i;
        i++;
        e[i].x = e[i-1].y,e[i].y = e[i-1].x,e[i].c = e[i-1].c,e[i].d = e[i-1].d;
        next[i] = first[e[i].x];
        first[e[i].x] = i;
        i++;
    }
    scanf(" %d %d %d",&s,&t,&k);
    distD[s] = distC[s] = 0;
    spfa(s);
    if(distD[t] == INF) printf("-1\n");
    else printf("%d\n",distD[t]);
}