bzoj1131: [POI2008]Sta

Posted 2020-11-21 sssy

题目链接

bzoj1131: [POI2008]Sta

题解

树形dp，我们可以预处理以1为根子树大小与ans，然后就可以O1转移

代码

/*
我们可以预处理以1为根子树大小与ans，然后就可以O1转移 
*/
#include<cstdio> 
#include<cstring> 
#include<algorithm>
const int maxn = 1000007;  
#define int long long 
struct node { 
     int v,next; 
} edge[maxn << 1]; 
int head[maxn],num = 0; 
inline void add_edge(int u,int v) { 
    edge[++ num].v = v;edge[num].next = head[u];head[u] = num; 
} 
int n,size[maxn],deep[maxn],sum[maxn] ; 
int ans = 0;   
void dfs(int x,int fa) { 
    size[x] = 1;  sum[x] = deep[x]; 
    for(int i = head[x];i;i = edge[i].next) { 
        //else top[edge[i].v] = top[x]; 
        if(edge[i].v != fa) { deep[edge[i].v] = deep[x] + 1; 
            dfs(edge[i].v,x); 
            sum[x] += sum[edge[i].v]; 
            size[x] += size[edge[i].v];  
        }    
    } 
} 
int pos = 1; 
int dp[maxn]; 
void Dfs(int x,int fa) { 
    for(int i = head[x];i;i = edge[i].next) { 
        int v = edge[i].v; 
        if(v == fa)continue;  
        dp[v] = dp[x] - size[v] + n - size[v]; 
        if(dp[v] > ans || (dp[v] == ans && v < pos)) ans = dp[v],pos = v;
        Dfs(v,x); 
    } 
} 
main() { 
    scanf("%lld",&n); 
    for(int u,v,i = 1;i < n;++ i) { 
         scanf("%lld%lld",&u,&v); 
         add_edge(u,v);add_edge(v,u); 
    } 
    dfs(1,0); 
    dp[1] = ans = sum[1]; 
    Dfs(1,0); 
    printf("%lld
",pos); 
    return 0;
}