自底向上贪心，优先删除每一个儿子中代价最小的一个。
正确性我也不是十分明白，画图的时候发现贪心删边都对，于是试一试竟然AC了……
1 #include<cstdio> 2 #include<iostream> 3 #include<cmath> 4 #include<algorithm> 5 #include<cstring> 6 #include<cstdlib> 7 #include<cctype> 8 #include<vector> 9 #include<stack> 10 #include<queue> 11 using namespace std; 12 #define enter puts("") 13 #define space putchar(' ') 14 #define Mem(a, x) memset(a, x, sizeof(a)) 15 #define rg register 16 typedef long long ll; 17 typedef double db; 18 const int INF = 0x3f3f3f3f; 19 const db eps = 1e-8; 20 const int maxn = 2e6 + 5; 21 inline ll read() 22 { 23 ll ans = 0; 24 char ch = getchar(), last = ' '; 25 while(!isdigit(ch)) {last = ch; ch = getchar();} 26 while(isdigit(ch)) {ans = ans * 10 + ch - '0'; ch = getchar();} 27 if(last == '-') ans = -ans; 28 return ans; 29 } 30 inline void write(ll x) 31 { 32 if(x < 0) x = -x, putchar('-'); 33 if(x >= 10) write(x / 10); 34 putchar(x % 10 + '0'); 35 } 36 37 int n, m; 38 39 struct Edge 40 { 41 int nxt, to; 42 }e[maxn << 1]; 43 int head[maxn], ecnt = -1; 44 void addEdge(int x, int y) 45 { 46 e[++ecnt] = (Edge){head[x], y}; 47 head[x] = ecnt; 48 } 49 50 int t[maxn], c[maxn], ans = 0; 51 void dfs(int now) 52 { 53 int cnt = 0; 54 for(int i = head[now]; i != -1; i = e[i].nxt) dfs(e[i].to); 55 for(int i = head[now]; i != -1; i = e[i].nxt) t[++cnt] = c[e[i].to]; 56 sort(t + 1, t + cnt + 1); 57 for(int i = 1; i <= cnt; ++i) 58 { 59 if(c[now] + t[i] - 1 > m) break; 60 c[now] += t[i] - 1, ans++; 61 } 62 } 63 64 int main() 65 { 66 Mem(head, -1); 67 n = read(); m = read(); 68 for(int i = 0; i < n; ++i) c[i] = read(); 69 for(int i = 0; i < n; ++i) 70 { 71 int k = read(); c[i] += k; 72 for(int j = 1; j <= k; ++j) 73 { 74 int x = read(); 75 addEdge(i, x); 76 } 77 } 78 dfs(0); 79 write(ans), enter; 80 return 0; 81 }