codeforces 682C Alyona and the Tree DFS
这个题就是在dfs的过程中记录到根的前缀和,以及前缀和的最小值
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
#include <cstdio> #include <iostream> #include <ctime> #include <vector> #include <cmath> #include <map> #include <stack> #include <queue> #include <algorithm> #include <cstring> using namespace std; typedef long long LL; const int N=1e5+5; const int INF=0x3f3f3f3f; const int mod=1e9+7; struct Edge{ int v,w,next; }edge[N<<1]; int head[N],tot,a[N],n; bool del[N]; void add(int u,int v,int w){ edge[tot].v=v; edge[tot].w=w; edge[tot].next=head[u]; head[u]=tot++; } int ret; void dfs(int u,int f,LL cur,LL mn){ if(del[f])del[u]=true,++ret; else if(cur-mn>a[u])del[u]=true,++ret; for(int i=head[u];~i;i=edge[i].next){ int v=edge[i].v; if(v==f)continue; dfs(v,u,cur+1ll*edge[i].w,min(cur+1ll*edge[i].w,mn)); } } int main(){ scanf("%d",&n); for(int i=1;i<=n;++i)scanf("%d",&a[i]); memset(head,-1,sizeof(head)); for(int i=2;i<=n;++i){ int u=i,v,w; scanf("%d%d",&v,&w); add(u,v,w);add(v,u,w); } dfs(1,0,0,0); printf("%d\n",ret); return 0; }