CodeForces 109C 树形DP Lucky Tree
赶脚官方题解写得挺清楚的说，=_=
注意数据范围用long long，否则会溢出。
1 #include <iostream> 2 #include <cstdio> 3 #include <algorithm> 4 #include <cstring> 5 #include <vector> 6 using namespace std; 7 8 const int maxn = 100000 + 10; 9 10 int n; 11 vector<int> G[maxn], C[maxn]; 12 13 inline int lucky(int x) 14 { 15 while(x) 16 { 17 if(x % 10 != 4 && x % 10 != 7) return 0; 18 x /= 10; 19 } 20 return 1; 21 } 22 23 long long d[maxn], sz[maxn], f[maxn]; 24 25 void dfs(int u, int fa) 26 { 27 sz[u] = 1; 28 f[u] = 0; 29 for(int i = 0; i < G[u].size(); i++) 30 { 31 int v = G[u][i]; 32 if(v == fa) continue; 33 dfs(v, u); 34 sz[u] += sz[v]; 35 if(C[u][i]) f[u] += sz[v]; 36 else f[u] += f[v]; 37 } 38 } 39 40 void dfs2(int u, int fa) 41 { 42 for(int i = 0; i < G[u].size(); i++) 43 { 44 int v = G[u][i]; 45 if(v == fa) continue; 46 if(C[u][i]) d[v] = sz[1] - sz[v]; 47 else d[v] = d[u] + f[u] - f[v]; 48 dfs2(v, u); 49 } 50 } 51 52 int main() 53 { 54 //freopen("in.txt", "r", stdin); 55 56 scanf("%d", &n); 57 for(int i = 1; i < n; i++) 58 { 59 int u, v, d; scanf("%d%d%d", &u, &v, &d); 60 int t = lucky(d); 61 G[u].push_back(v); C[u].push_back(t); 62 G[v].push_back(u); C[v].push_back(t); 63 } 64 65 dfs(1, 0); 66 dfs2(1, 0); 67 68 long long ans = 0; 69 for(int i = 1; i <= n; i++) ans += f[i] * (f[i] - 1) + d[i] * (d[i] - 1) + 2 * f[i] * d[i]; 70 printf("%I64d\n", ans); 71 72 return 0; 73 }
