HDU 3249 Test for job （有向无环图上的最长路，DP）

Posted 2020-09-21

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解题思路：

求有向无环图上的最长路。简单的动态规划

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <cmath>
#define LL long long
using namespace std;
const int MAXN = 100000 + 10;
const int MAXM = 1000000 + 10;
const LL INF = -1 * 20000 * 100000 - 10;
struct Edge
{
    int to, next;
}edge[MAXM];
int tot, head[MAXN];
LL w[MAXN];
LL dp[MAXN];
int out[MAXN], vis[MAXN];
int n, m;
void init()
{
    tot = 0;
    memset(head, -1, sizeof(head));
    memset(w, 0, sizeof(w));
    memset(dp, INF, sizeof(dp));
    memset(vis, 0, sizeof(vis));
    memset(out, 0, sizeof(out));
}
void addedge(int u, int v)
{
    edge[tot].to = v;
    edge[tot].next = head[u];
    head[u] = tot++;
}
int dfs(int u)
{
    //cout << u << endl;
    if(vis[u]) return dp[u];
    vis[u] = 1;
    for(int i=head[u];i!=-1;i=edge[i].next)
    {
        int v = edge[i].to;
        dp[u] = max(dp[u], (LL)dfs(v));
    }
    if(head[u] == -1) return dp[u] = w[u];
    else return dp[u] = (dp[u] + w[u]);
   // return dp[u];
}
int main()
{
    while(scanf("%d%d", &n, &m)!=EOF)
    {
        init();
        for(int i=1;i<=n;i++) scanf("%d", &w[i]);
        int u, v;
        for(int i=1;i<=m;i++)
        {
            scanf("%d%d", &u, &v);
            addedge(v, u);
            out[u]++;
        }
        LL ans = INF;
        //for(int i=1;i<=n;i++) cout << dp[i] << ‘ ‘;
        for(int i=1;i<=n;i++) if(!out[i])ans = max(ans, (LL)dfs(i));
        printf("%d\n", ans);
    }
    return 0;
}