BZOJ2435 [Noi2011]道路修建

 1 /**************************************************************
 2     Problem: 2435
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:5184 ms
 7     Memory:76756 kb
 8 ****************************************************************/
 9  
10 #include <cstdio>
11  
12 using namespace std;
13 typedef long long ll;
14 const int N = 1000005;
15  
16 struct edge {
17     int next, to, v;
18     edge() {}
19     edge(int _n, int _t, int _v) : next(_n), to(_t), v(_v) {}
20 } e[N << 1];
21  
22 struct tree_node {
23     int vis, sz;
24 } tr[N];
25  
26 int first[N], tot;
27 int n;
28 ll ans;
29  
30 inline int read() {
31     int x = 0;
32     char ch = getchar();
33     while (ch < ‘0‘ || ‘9‘ < ch)
34         ch = getchar();
35     while (‘0‘ <= ch && ch <= ‘9‘) {
36         x = x * 10 + ch - ‘0‘;
37         ch = getchar();
38     }
39     return x;
40 }
41  
42 void Add_Edges(int x, int y, int z) {
43     e[++tot] = edge(first[x], y, z), first[x] = tot;
44     e[++tot] = edge(first[y], x, z), first[y] = tot;
45 }
46  
47 inline int abs(int x) {
48     return x < 0 ? -x : x;
49 }
50  
51 #define y e[x].to
52 void dfs(int p) {
53     int x;
54     tr[p].vis = tr[p].sz = 1;
55     for (x = first[p]; x; x = e[x].next)
56         if (!tr[y].vis) {
57             dfs(y);
58             tr[p].sz += tr[y].sz;
59             ans += 1ll * abs(n - (tr[y].sz << 1)) * e[x].v;
60         }
61 }
62 #undef y
63  
64 int main() {
65     int i, x, y, z;
66     n = read();
67     for (i = 1; i < n; ++i) {
68         x = read(), y = read(), z = read();
69         Add_Edges(x, y, z);
70     }
71     dfs(1);
72     printf("%lld\n", ans);
73     return 0;
74 }