CF796C Bank Hacking 细节

Posted 2022-12-17 guangheli

思路十分简单，答案只有 3 种可能，但是有一些细节需要额外注意一下. 

code: 

#include <bits/stdc++.h>
#define N 300002    
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std; 
int val[N],hd[N],to[N<<1],nex[N<<1],d1[N],d2[N],n,edges,maxx,mx,m2,cnt,uu;             
void add(int u,int v) 

    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v; 
    
void dfs(int u,int ff) 

    if(val[u]==mx) d1[u]=0, uu=u;         
    for(int i=hd[u];i;i=nex[i]) 
    
        int v=to[i]; 
        if(v==ff) continue;   
        dfs(v,u);    
        if(d1[v]+1>d1[u]) 
        
            d2[u]=d1[u],d1[u]=d1[v]+1; 
        
        else if(d1[v]+1>d2[u]) d2[u]=d1[v]+1;    
    
    maxx=max(d1[u]+d2[u], maxx); 

int main() 
 
    int i,j; 
    // setIO("input");     
    mx=-1000300000; 
    m2=mx;   
    scanf("%d",&n);    
    for(i=1;i<=n;++i) 
    
        scanf("%d",&val[i]),mx=max(mx,val[i]);   
    
    for(i=1;i<=n;++i) if(val[i]<mx) m2=max(m2, val[i]); 
    for(i=1;i<=n;++i) if(val[i]==m2) ++cnt;    
    for(i=1;i<n;++i) 
    
        int u,v;      
        scanf("%d%d",&u,&v),add(u,v),add(v,u); 
       
    memset(d1,-0x3f,sizeof(d1));         
    memset(d2,-0x3f,sizeof(d2));             
    dfs(1,0);         
    if(maxx==0) 
     
        if(m2!=mx-1) 
            printf("%d\n",mx);          
        else 
           
            for(int i=hd[uu];i;i=nex[i])
            
                int v=to[i]; 
                if(val[v]==m2) --cnt;  
            
            if(cnt) printf("%d\n",mx+1); 
            else printf("%d\n",mx);   
        
    
    else if(maxx<=2) printf("%d\n",mx+1);                        
    else printf("%d\n",mx+2); 
    return 0;