Rebuilding Roads

#include <iostream>
#include <algorithm>
#include <string>
#include <string.h>
#include <vector>
#include <map>
#include <stack>
#include <set>
#include <queue>
#include <math.h>
#include <cstdio>
#include <iomanip>
#include <time.h>
#include <bitset>
#include <cmath>

#define LL long long
#define INF 0x3f3f3f3f
#define ls nod<<1
#define rs (nod<<1)+1

const double eps = 1e-10;
const int maxn = 3000 + 10;
const LL mod = 1e9 + 7;

int sgn(double a){return a < -eps ? -1 : a < eps ? 0 : 1;}
using namespace std;

struct edge {
    int v,nxt;
}e[maxn<<1];

int head[maxn];
int cnt;
int f[maxn][maxn];
int root[maxn];
int n,m;

inline void add_edge(int u,int v) {
    e[++cnt].v = v;
    e[cnt].nxt = head[u];
    head[u] = cnt;
}

inline void dfs(int x) {
    f[x][1] = 0;
    for (int i = head[x];~i;i = e[i].nxt) {
        int v = e[i].v;
        dfs(v);
        for (int j = m;j >= 1;j--) {
            f[x][j] = f[x][j] + 1;
            for (int k = 1;k < j;k++) {
                f[x][j]=min(f[x][j],f[v][k]+f[x][j-k]);
            }
        }
    }
}

int main() {
    while (cin >> n >> m) {
        cnt = 0;
        memset(head,-1, sizeof(head));
        for (int i = 1;i <= n;i++) {
            for (int j = 0;j <= m;j++)
                f[i][j] = INF;
        }
        for (int i = 1;i < n;i++) {
            int u,v;
            cin >> u >> v;
            add_edge(u,v);
            root[v] = 1;
        }
        int ans = 0;
        for (int i = 1;i <= n;i++) {
            if (!root[i]) {
                dfs(i);
                ans = f[i][m];
            }
        }
        for (int i = 1;i <= n;i++) {
            ans = min(ans,f[i][m]+1);   // 如果当前节点不是真正的根节点，那么还要去掉他和他父亲之间的一条边
        }
        cout << ans << endl;
    }
    return 0;
}