hdu 5877 Weak Pair（dfs+树状数组）

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问你一棵树上有多少对(u,v)，u是v的祖先并且au∗av≤k
如果你考虑u的子树上的东西，可能比较麻烦
所以直接在dfs下去的时候，因为这会搜下去的一条路上，前面的节点肯定都是u的祖先
所以直接把ai放进去，然后每个点查一下小于等于kai
然后回溯上去的时候再去掉就行了

代码：

#include <map>#include <set>#include <stack>#include <queue>#include <cmath>#include <string>#include <vector>#include <cstdio>#include <cctype>#include <cstring>#include <sstream>#include <cstdlib>#include <iostream>#include <algorithm>#pragma comment(linker,"/STACK:102400000,102400000")using namespace std;#define   MAX           100005#define   MAXN          1000005#define   maxnode       15#define   sigma_size    30#define   lson          l,m,rt<<1#define   rson          m+1,r,rt<<1|1#define   lrt           rt<<1#define   rrt           rt<<1|1#define   middle        int m=(r+l)>>1#define   LL            long long#define   ull           unsigned long long#define   mem(x,v)      memset(x,v,sizeof(x))#define   lowbit(x)     (x&-x)#define   pii           pair<int,int>#define   bits(a)       __builtin_popcount(a)#define   mk            make_pair#define   limit         10000//const int    prime = 999983;const int    INF   = 0x3f3f3f3f;const LL     INFF  = 0x3f3f;const double pi    = acos(-1.0);const double inf   = 1e18;const double eps   = 1e-8;const int    mod   = 1e9+7;const ull    mx    = 133333331;/*****************************************************/inline void RI(int &x) {      char c;      while((c=getchar())<'0' || c>'9');      x=c-'0';      while((c=getchar())>='0' && c<='9') x=(x<<3)+(x<<1)+c-'0'; }/*****************************************************/struct Edge{    int v,next;}edge[MAX];int head[MAX];int degree[MAX];LL a[MAX];LL b[MAX];LL x[MAX*2];int tot;int cnt;int sum[MAX*2];LL ans;void init(){    mem(head,-1);    mem(degree,0);    mem(sum,0);    tot=0;}void add_edge(int a,int b){    edge[tot]=(Edge){b,head[a]};    head[a]=tot++;}int query(int x){    int ret=0;    while(x){        ret+=sum[x];        x-=lowbit(x);    }    return ret;}void add(int x,int d){    while(x<=cnt){        sum[x]+=d;        x+=lowbit(x);    }}void dfs(int u){    int pos=lower_bound(x,x+cnt,b[u])-x+1;    int xx=lower_bound(x,x+cnt,a[u])-x+1;    ans+=query(pos);    add(xx,1);    for(int i=head[u];i!=-1;i=edge[i].next){        int v=edge[i].v;        dfs(v);    }    add(xx,-1);}int main(){    //freopen("in.out","r",stdin);    int t;    cin>>t;    while(t--){        int n;        LL k;        cin>>n>>k;        init();        cnt=0;        for(int i=1;i<=n;i++){            scanf("%I64d",&a[i]);            x[cnt++]=a[i];            if(a[i]==0) b[i]=1e18+5;            else b[i]=k/a[i];            x[cnt++]=b[i];        }        sort(x,x+cnt);        cnt=unique(x,x+cnt)-x;        for(int i=1;i<n;i++){            int aa,b;            scanf("%d%d",&aa,&b);            add_edge(aa,b);            degree[b]++;        }        int root=0;        for(int i=1;i<=n;i++){            if(!degree[i]) root=i;        }        ans=0;        dfs(root);        cout<<ans<<endl;    }    return 0;}

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