BZOJ3436 小K的农场

首先这是个查分约束系统。。。（那是啥来着←_←）

然后判断可行解的话，直接spfa判负圈。

bfs版spfa要记录每个点的进队次数，如果大于点数就代表有负圈。。。但是好慢Σ( ° △ °|||)︴

于是改用dfs版的spfa（还以为会很慢。。。，其实速度极快Orz）

 

 1 /**************************************************************
 2     Problem: 3436
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:36 ms
 7     Memory:1236 kb
 8 ****************************************************************/
 9  
10 #include <cstdio>
11  
12 using namespace std;
13 const int N = 10005;
14 const int M = N;
15 const int Maxlen = 20 * M;
16  
17 struct edges {
18     int next, to, v;
19     edges() {}
20     edges(int _n, int _t, int _v) : next(_n), to(_t), v(_v) {}
21 } e[M];
22  
23 int n, m;
24 int first[N], tot;
25 int d[N];
26 int v[N], flag;
27 char buf[Maxlen], *c = buf;
28 int Len;
29  
30 inline int read() {
31     int x = 0;
32     while (*c < '0' || '9' < *c) ++c;
33     while ('0' <= *c && *c <= '9')
34         x = x * 10 + *c - '0', ++c;
35     return x;
36 }
37  
38 inline void add_edge(int x, int y, int z) {
39     e[++tot] = edges(first[x], y, z);
40     first[x] = tot;
41 }
42  
43 void spfa(int p) {
44     if (v[p]) {
45         flag = 1;
46         return;
47     }
48     int x, y;
49     v[p] = 1;
50     for (x = first[p]; x; x = e[x].next)
51         if (d[p] + e[x].v < d[y = e[x].to]) {
52             d[y] = d[p] + e[x].v;
53             spfa(y);
54             if (flag) return;
55         }
56     v[p] = 0;
57 }
58  
59 int main() {
60     Len = fread(c, 1, Maxlen, stdin);
61     buf[Len] = '\0';
62     int i, oper, x, y, z;
63     n = read(), m = read();
64     for (i = 1; i <= m; ++i) {
65         oper = read(), x = read(), y = read();
66         if (oper == 3) add_edge(x, y, 0);
67         else {
68             z = read();
69             if (oper == 1) add_edge(x, y, -z);
70             else add_edge(y, x, z);
71         }
72     }
73     for (i = 1; i <= n; ++i) {
74         spfa(i);
75         if (flag) break;
76     }
77     puts(flag ? "No" : "Yes");
78     return 0;
79 }
View Code

 

posted on 2014-11-30 17:32  Xs酱~  阅读(217)  评论(0)  编辑  收藏  举报