Comet OJ - Contest #9 & X Round 3 【XR-3】核心城市 【树

#include <bits/stdc++.h>

using namespace std;
#define ll long long
#define Min(a,b) ((a)>(b)?(b):(a))
#define Max(a,b) ((a)>(b)?(a):(b))
const int MAXN = 1e5;
vector<int> G[MAXN + 13];
int Depth[MAXN + 13],Num[MAXN + 13];

int BFS(int n,int k)
{
    memset(Depth,0,sizeof(Depth));
    queue<int> que;
    for(int i = 1; i <= n; i++) {
        if(G[i].size() == 1) {
            Depth[i] = 1;
            que.push(i);
        }
    }
    int last = n;
    while(!que.empty()) {
        int u = que.front();
        que.pop();
        last--;
        if(last == k)   return Depth[u];
        for(int i = 0; i < G[u].size(); i++) {
            int v = G[u][i];
            Num[v]--;
            if(Num[v] == 1) {
                Depth[v] = Max(Depth[v],Depth[u] + 1);
                que.push(v);
            }
        }
    }
    return 0;
}

void init(int n)
{
    memset(Num,sizeof(Num));
    for(int i = 1; i <= n; i++) {
        G[i].clear();
    }
}

int main()
{
    //freopen("input.txt","r",stdin);
    // freopen("out.txt","w",stdout);
    int n,k;
    while(scanf("%d%d",&n,&k)!=EOF) {
        int u,v;
        init(n);
        for(int i = 1; i < n; i++) {
            scanf("%d%d",&u,&v);
            G[u].push_back(v);
            G[v].push_back(u);
            Num[u]++,Num[v]++;
        }
        printf("%dn",BFS(n,k));
        // cout << solve(n,k) << endl;

    }
    return 0;
}