P1219 最佳贸易

Posted 2021-01-29 ak-ls

AC这道题已经是第二遍了，第一次是用别人一个叫DP的代码水过的，尽管我都不知道什么事DP;今天突然有了灵感，自己又写了一遍，用双向SPFA，第一次就过了，和大家分享一下；

#include <bits/stdc++.h>

using namespace std;

#define N 100000

inline int read()
{
    int x = 0,ff = 1;char ch = getchar();
    while(!isdigit(ch))
    {
        if(ch == ‘-‘) ff = -1;
        ch = getchar();
    }
    while(isdigit(ch))
    {
        x = (x<<1) + (x<<3) + (ch ^ 48);
        ch = getchar();
    }
    return x * ff;
}

inline void write(int x)
{
    if(x < 0) putchar(‘-‘),x = -x;
    if(x > 9) write(x / 10);
    putchar(x % 10 + ‘0‘);
}

int a,b,ans;
struct node
{
    int y,next;
}e1[N<<1],e2[N<<1];
int tot1,lin1[N<<1];
int tot2,lin2[N<<1];
int mon[N<<1],mi[N<<1],ma[N<<1];
int head,tail,q[N<<1],vis[N<<1];

void add1(int xx,int yy)
{
    e1[++tot1].y = yy;
    e1[tot1].next = lin1[xx];
    lin1[xx] = tot1;
}

void add2(int xx,int yy)
{
    e2[++tot2].y = yy;
    e2[tot2].next = lin2[xx];
    lin2[xx] = tot2;
}

void SPFA1(int s)
{
    memset(vis,false,sizeof(vis));
    memset(mi,0x3f,sizeof(mi));
    head = 1,tail = 1;
    q[1] = s; mi[s] = mon[s];
    for(head = 1;head <= tail;++head)
    {
        int x = q[head];
        vis[x] = false;
        for(int i = lin1[x],y;i;i = e1[i].next)
        {
            if(mi[y = e1[i].y] > mi[x])
            {
                mi[y] = min(mon[y],mi[x]);
                if(!vis[y])
                {
                    vis[y] = true;
                    q[++tail] = y;
                }
            }
        }
    }
}

void SPFA2(int s)
{
    memset(vis,false,sizeof(vis));
    head = 1,tail = 1;
    q[1] = s; ma[s] = mon[s];
    for(head = 1;head <= tail;++head)
    {
        int x = q[head];
        vis[x] = false;
        for(int i = lin2[x],y;i;i = e2[i].next)
        {
            if(ma[y = e2[i].y] < ma[x])
            {
                ma[y] = max(mon[y],ma[x]);
                if(!vis[y])
                {
                    vis[y] = true;
                    q[++tail] = y;
                }
            }
        }
    }
}

int main()
{
    a = read(); b = read();
    for(int i = 1;i <= a;++i)
    mon[i] = read();
    for(int i = 1;i <= b;++i)
    {
        int x,y,z;
        x = read(); y = read(); z = read();
        add1(x,y);
        add2(y,x);
        if(z == 2)
        {
            add1(y,x);
            add2(x,y);
        }
    }
    SPFA1(1); SPFA2(a);
    for(int i = 1;i <= a;++i)
    ans = max(ans,ma[i] - mi[i]);
    write(ans); 
    return 0;
}

下面是第一次的代码，大佬可以参考一下......

#include<bits/stdc++.h>
#define INF 0x7f7f7f7f
#define MAXN 100005
using namespace std;

vector<int> g[MAXN];
int n,m,f[MAXN],mi[MAXN],c[MAXN];

void dfs(int x,int minx,int pre) {
    int flag=1; 
    minx=min(c[x],minx);
    if (mi[x]>minx) mi[x]=minx,flag=0;
    int maxx=max(f[pre],c[x]-minx);
    if (f[x]<maxx) f[x]=maxx,flag=0;
    if (flag) return;
    for (int i=0;i<g[x].size();i++) dfs(g[x][i],minx,x);
}

int main() {
    scanf("%d%d",&n,&m);
    for (int i=0;i<MAXN;i++) mi[i]=INF;
    for (int i=1;i<=n;i++) scanf("%d",&c[i]);
    for (int i=1;i<=m;i++) {
        int t1,t2,t3;
        scanf("%d%d%d",&t1,&t2,&t3);
        g[t1].push_back(t2);
        if (t3==2) g[t2].push_back(t1);
    }
    dfs(1,INF,0);
    printf("%d
",f[n]);
    return 0;
}