Codeforces Gym102460 B-The Power Monitor System （树上

Posted 2020-11-27 cminus

我无法形容出这道题的题解，实在是太麻烦了，但是自己开的题，跪着也要做完，放个std仅供对拍

#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>
using namespace std;
const int N = 100010;
vector < int > E[N];
int fa[N], d[N][3][3];
void dfs(int u) {
    int tot1 = 0, tot2 = 0, maxn = -N, maxn2 = -N, maxn3 = -N;
    if (E[u].size() == 1 && u != 1) {
        d[u][0][0] = 0;
        d[u][1][1] = 1;
        d[u][1][0] = 1;
        return ;
    }
    if (u == 1 && E[u].size() == 1) {
        fa[E[u][0]] = u;
        dfs(E[u][0]);
        d[u][1][0] = d[u][1][1] = min(d[E[u][0]][1][1], d[E[u][0]][0][0] + 1);
        return ;
    }
    if (E[u].size() == 2 && u != 1) {
        int v;
        if (fa[u] == E[u][0])
            v = E[u][1];
        else
            v = E[u][0];
        fa[v] = u;
        dfs(v);
        d[u][1][0] = d[u][1][1] = min(d[v][1][1], d[v][0][0] + 1);
        d[u][0][0] = min(d[v][1][0], d[v][0][0]);
        return ;
    }
    for (int i = 0; i < E[u].size(); i++) {
        int v = E[u][i];
        if (v == fa[u])    continue;
        fa[v] = u;
        dfs(v);
        tot1 += d[v][1][0];
        tot2 += d[v][0][0];
        maxn = max(maxn, d[v][1][0] - d[v][1][1] + maxn2);
        maxn = max(maxn, d[v][1][0] - d[v][0][0] + maxn3);
        maxn2 = max(maxn2, d[v][1][0] - d[v][0][0]);
        maxn3 = max(maxn3, d[v][1][0] - d[v][1][1]);
    }
    d[u][0][0] = tot1 - maxn2;
    d[u][1][0] = tot1 - maxn;
    d[u][1][1] = min(tot2 + 1, tot1 - maxn3);
    d[u][1][0] = min(d[u][1][0], d[u][1][1]);
    d[u][0][0] = min(d[u][0][0], d[u][1][0]);
}
int main() {
    int n;
    scanf("%d", &n);
    memset(d, 0x3f, sizeof(d));
    for (int i = 1; i < n; i++) {
        int u, v;
        scanf("%d %d", &u, &v);
        E[u].push_back(v);
        E[v].push_back(u);
    }
    dfs(1);
    //for (int i = 1; i <= n; i++)
    //    printf("%d %d %d
", d[i][1][0], d[i][1][1], d[i][0][0]);
    printf("%d", min(d[1][1][0], d[1][1][1]));
    return 0;
}