洛谷P3018 [USACO11MAR]樹裝飾Tree Decoration
樹形DP
因為要求最小，我們就貪心地用每個子樹中的最小cost來支付就行了

 1 #include <bits/stdc++.h>
 2 #define For(i, j, k) for(int i=j; i<=k; i++)
 3 #define Dow(i, j, k) for(int i=j; i>=k; i--)
 4 #define LL long long
 5 using namespace std;
 6 inline int read() {
 7     int x = 0
 
, f = 1;
 8     char ch = getchar();
 9     while(ch<‘0‘||ch>‘9‘) { if(ch==‘-‘) f = -1; ch = getchar(); }
10     while(ch>=‘0‘&&ch<=‘9‘) { x = x*10+ch-48; ch = getchar(); }
11     return x * f;
12 }
13 
14 const int N = 1e5+11; 
15 int n, nedge; 
16 int Mn[N], fa[N], head[N], need[N]; 
 
17 LL f[N], have[N];  
18 struct edge{
19     int to, pre; 
20 }e[N*2];
21 
22 inline void add(int x,int y) {
23     e[++nedge].to = y; 
24     e[nedge].pre = head[x]; 
25     head[x] = nedge; 
26 }
27 
28 void dfs(int u) {
29     for(int i=head[u]; i; i=e[i].pre) {
30         int v = e[i].to; 
31         if
 
(v == fa[u]) continue; 
32         dfs( v ); 
33         have[u] += have[v]; 
34         Mn[u] = min(Mn[u], Mn[v]); 
35         f[u] += f[v]; 
36     }
37     if(need[u] > have[u]) {
38         f[u] = f[u]+1ll*(need[u]-have[u])*Mn[u]; 
39         have[u] = need[u]; 
40     }
41 }
42 
43 int main() {
44     n = read(); 
45     For(i, 1, n) {
46         fa[i] = read(); 
47         need[i] = read(); 
48         Mn[i] = read(); 
49         if(fa[i] != -1) add(i, fa[i]), add(fa[i], i); 
50     }
51     dfs(1); 
52     printf("%lld\n",f[1]); 
53     return 0; 
54 }

洛谷P3018 [USACO11MAR]樹裝飾Tree Decoration