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 + 5;
typedef long long LL;
struct Edge {
    int next, to;
}edge[N * 10];
LL dp[N], a[N], inf = 1e12;
int head[N], tot, input[N];

void init(int n) {
    for(int i = 1; i <= n; ++i) {
        head[i] = -1;
        dp[i] = -inf;
        input[i] = 0;
    }
    tot = 0;
}

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 = 1; i <= n; ++i) {
            scanf("%lld", a + i);
        }
        init(n);
        int u, v;
        for(int i = 1; i <= m; ++i) {
            scanf("%d %d", &u, &v);
            add(u, v);
            input[v]++;
        }
        LL Max = -inf;
        for(int i = 1; i <= n; ++i) {
            if(!input[i]) {
                dfs(i);
                Max = max(Max, dp[i]);
            }
        }
        printf("%lld
", Max);
    }
    return 0;
}