LiberOJ #6210. 「美团 CodeM 决赛」tree 树形DP

#include<bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:102400000,102400000")
#define ls i<<1
#define rs ls | 1
#define mid ((ll+rr)>>1)
#define pii pair<int,int>
#define MP make_pair
typedef long long LL;
const long long INF = 1e18+1LL;
const double pi = acos(-1.0);
const int N = 5e5+10, M = 1e3+20,inf = 2e9;
const LL mod = 1e9+7LL;

int n,a[N],ans;
int ans1,ans2;
vector<int > G[N];
int dp[N][2];
void dfs(int u,int f) {
    if(a[u] == 1) dp[u][0] = 1;
    else if(a[u] == 2) dp[u][1] = 1;
    for(int i = 0; i < G[u].size(); ++i) {
        int to = G[u][i];
        if(to == f) continue;
        dfs(to,u);
        ans1 = max(ans1,dp[to][0] + dp[u][0]);
        ans2 = max(ans2,dp[u][0] + dp[to][1]);
        ans2 = max(ans2,dp[u][1] + dp[to][0]);
        if(a[u] > 2)
            continue;
            
        if(a[u] == 1) {
            dp[u][0] = max(dp[u][0],dp[to][0] + 1);
            dp[u][1] = max(dp[u][1],dp[to][1] + 1);
        }
        else {
            dp[u][1] = max(dp[u][1],dp[to][0] + 1);
        }
    }
}
int main(){
    scanf("%d",&n);
    for(int i = 1; i < n; ++i) {
        int x,y;
        scanf("%d%d",&x,&y);
        G[x].push_back(y);
        G[y].push_back(x);
    }
    int mi = inf;
    for(int i = 1; i <= n; ++i) {
        scanf("%d",&a[i]);
        mi = min(mi,a[i]);
    }
    ans = 0;
    dfs(1,0);
    if(mi >= 2) {
        printf("%d/1\n",mi);
    }
    else {
        if(2*ans1 > ans2) printf("1/%d\n",ans1);
        else if(ans2 % 2 == 0) printf("1/%d\n",ans2/2);
        else printf("2/%d\n",ans2);
    }
    return 0;
}