codeforces 682C Alyona and the Tree DFS

Posted 2020-07-29 shuguangzw

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

#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;
}

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