【樹形dp】Codeforces 238C World Eater Brothers

題目點這裏

嗯。。。其實就直接dfs就可以了。。。

就是先枚舉一條邊砍掉 = =

然後呢先計算 u 和 v 的答案。。

然後再dfs一遍修正答案。。。。。

#include <cstdio>
#include <iostream>

using namespace std;

const int Nmax = 3e3 + 5;

int N;
struct ed{
    int u, v, w, next;
}e[Nmax * 2];
int k = 1, head[Nmax];

inline void adde(int u, int v, int w)
{
    e[k] = (ed) { u, v, w, head[u] };
    head[u] = k++;
}

int pre(int u, int fa)
{
    int res = 0;
    for (int i = head[u]; i; i = e[i].next) {
        if (e[i].v == fa) continue;
        res += pre(e[i].v, u);
        if (!e[i].w) ++ res;
    }
    return res;
}

int find(int u, int fa, int already)
{
    int res = already;
    for (int i  = head[u]; i; i = e[i].next) {
        if (e[i].v == fa) continue;
        int next = already;
        if (e[i].w) ++ next; else -- next;
        res = min(res, find(e[i].v, u, next));
    }
    return res;
}

int main()
{
    ios :: sync_with_stdio(false);
    cin >> N; int u, v;
    for (int i = 1; i < N; ++ i) {
        cin >> u >> v;
        adde(u, v, 1); adde(v, u, 0);
    }
    
    int ans = N - 1; 
    for (int i = 1; i < k; i += 2) {
        int u = e[i].u, v = e[i].v;
        int cnt1 = pre(u, v), cnt2 = pre(v, u);
        //cout << cnt1 << " " << cnt2 << endl;
        cnt1 = find(u, v, cnt1); cnt2 = find(v, u, cnt2);
        //cout << cnt1 << " " << cnt2 << endl;
        ans = min(ans, cnt1 + cnt2);
    }
    cout << ans << endl;
    
    return 0;
}