洛谷P1041 传染病控制

解：搜索......

我们可以每次选择分支少的搜索，或者说，贪心的搜索当前更优的决策。

每一层把能剪的点搞出来，按照度数/SIZ排序，然后依次搜索。加个最优化剪枝就完事了。

#include <bits/stdc++.h>

const int N = 310, INF = 0x3f3f3f3f;

struct Edge {
    int nex, v;
}edge[N << 1]; int tp;

int e[N], siz[N], in[N], n, ans = INF;
int v[N][N];
bool del[N], vis[N];

inline bool cmp(const int &a, const int &b) {
    //return in[a] > in[b];
    return siz[a] > siz[b];
}

inline void add(int x, int y) {
    tp++;
    edge[tp].v = y;
    edge[tp].nex = e[x];
    e[x] = tp;
    return;
}

void DFS_1(int x, int f) {
    siz[x] = 1;
    for(int i = e[x]; i; i = edge[i].nex) {
        int y = edge[i].v;
        if(y == f) continue;
        DFS_1(y, x);
        siz[x] += siz[y];
    }
    return;    
}

void DFS(int k, int d) {
    if(k >= ans) return;
    v[d][0] = 0;
    for(int x = 1; x <= n; x++) {
        if(del[x] || !vis[x]) continue;
        /// vis[x] 
        for(int i = e[x]; i; i = edge[i].nex) {
            int y = edge[i].v;
            if(vis[y] || del[y]) continue;
            v[d][++v[d][0]] = y;
        }
    }
    
    if(!v[d][0]) {
        ans = std::min(ans, k);
        return;
    }
    
    std::sort(v[d] + 1, v[d] + v[d][0] + 1, cmp);
    for(int i = 1; i <= v[d][0]; i++) {
        vis[v[d][i]] = 1;
    }
    k += v[d][0] - 1;
    for(int i = 1; i <= v[d][0]; i++) {
        int x = v[d][i];
        del[x] = 1;
        vis[x] = 0;
        DFS(k, d + 1);
        vis[x] = 1;
        del[x] = 0;
    }
    for(int i = 1; i <= v[d][0]; i++) {
        vis[v[d][i]] = 0;
    }
    return;
}

int main() {
    
//    freopen("disease.in", "r", stdin);
//    freopen("disease.out", "w", stdout);

    int m;
    scanf("%d%d", &n, &m);
    for(int i = 1, x, y; i <= m; i++) {
        scanf("%d%d", &x, &y);
        add(x, y); add(y, x);
        in[x]++; in[y]++;
    }
    DFS_1(1, 0);
    vis[1] = 1;
    DFS(1, 0);
    
    printf("%d
", ans);
    return 0;
}