POJ 3249 Test for Job (dfs + dp)

题目链接：http://poj.org/problem?id=3249

题意:

给你一个DAG图，问你入度为0的点到出度为0的点的最长路是多少

思路：

记忆化搜索，注意v[i]可以是负的，所以初始值要-inf。

 #include <iostream>
 #include <cstdio>
 #include <cstring>
 using namespace std;
 const int N = 1e5 + ;
 typedef long long LL;
 struct Edge {
     int next, to;
 }edge[N * ];
 LL dp[N], a[N], inf = 1e12;
 int head[N], tot, input[N];

 void init(int n) {
     for(int i = ; i <= n; ++i) {
         head[i] = -;
         dp[i] = -inf;
         input[i] = ;
     }
     tot = ;
 }

 inline void add(int u, int v) {
     edge[tot].next = head[u];
     edge[tot].to = v;
     head[u] = tot++;
 }

 void dfs(int u) {
     if(dp[u] != -inf)
         return ;
     for(int i = head[u]; ~i; i = edge[i].next) {
         int v = edge[i].to;
         dfs(v);
         dp[u] = max(dp[v] + a[u], dp[u]);
     }
     if(dp[u] == -inf) //若是出度为0
         dp[u] = a[u];
 }

 int main()
 {
     int n, m;
     while(~scanf("%d %d", &n, &m)) {
         for(int i = ; i <= n; ++i) {
             scanf("%lld", a + i);
         }
         init(n);
         int u, v;
         for(int i = ; i <= m; ++i) {
             scanf("%d %d", &u, &v);
             add(u, v);
             input[v]++;
         }
         LL Max = -inf;
         for(int i = ; i <= n; ++i) {
             if(!input[i]) {
                 dfs(i);
                 Max = max(Max, dp[i]);
             }
         }
         printf("%lld\n", Max);
     }
     return ;
 }