【DP】 cf 486D

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考虑每个点作为最小的权值点。。。然后dp

#include <bits/stdc++.h>using namespace std;typedef long long LL;const int mod = 1000000007;const int maxn = 2005;const int maxm = 4005;struct Edge{int v;Edge *next;}E[maxm], *H[maxn], *edges;LL f[maxn];int a[maxn];int n, m;void addedges(int u, int v){edges->v = v;edges->next = H[u];H[u] = edges++;}void init(){edges = E;memset(H, 0, sizeof H);}void dfs(int u, int fa, int rt){f[u] = 1;for(Edge *e = H[u]; e; e = e->next) if(e->v != fa) {int v = e->v;if(a[v] < a[rt] || a[v] > a[rt] + m) continue;if(a[v] == a[rt] && v < rt) continue;dfs(v, u, rt);f[u] = f[u] * (f[v] + 1) % mod;}}void work(){scanf("%d%d", &m, &n);for(int i = 1; i <= n; i++) scanf("%d", &a[i]);for(int i = 1; i < n; i++) {int u, v;scanf("%d%d", &u, &v);addedges(u, v);addedges(v, u);}LL ans = 0;for(int i = 1; i <= n; i++) {dfs(i, i, i);ans = (ans + f[i]) % mod;}printf("%I64d\n", ans);}int main(){//freopen("data", "r", stdin);init();work();return 0;}

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