hdu 5483 Nux Walpurgis(最小生成树+dfs)

题目链接：hdu 5483 Nux Walpurgis

解题思路

先求一下最小生成树。然后枚举起点，遍历整棵树，维护树边能被替换的最小权值。

代码

#pragma comment(linker, "/STACK:102400000,102400000")
#include 
#include 
#include 
#include 

using namespace std;
const int maxn = 3005;
const int inf = 0x3f3f3f3f;

struct Edge {
    int u, v, w;
    Edge(int u = 0, int v = 0, int w = 0): u(u), v(v), w(w) {}
    bool operator < (const Edge& a) const { return w < a.w; }
};

int N, F[maxn], W[maxn], T[maxn];
int E, first[maxn], jump[maxn<<1], linker[maxn<<1], idx[maxn<<1];
vector S;
vector<int> D[maxn];

void addEdge(int u, int v, int w) {
    jump[E] = first[u];
    linker[E] = v;
    idx[E] = w;
    first[u] = E++;
}

int find (int x) { return x == F[x] ? x : F[x] = find(F[x]); }

void set(int u, int v, int w) {
    if (u > v) swap(u, v);
    D[u][v-u-1] = w;
}

int get (int u, int v) {
    if (u > v) swap(u, v);
    return D[u][v-u-1];
}

void init () {
    S.clear();
    scanf("%d", &N);

    int x;
    for (int i = 0; i < N; i++) {
        D[i].clear();
        for (int j = 1; j < N-i; j++) {
            scanf("%d", &x);
            D[i].push_back(x);
            S.push_back(Edge(i, i+j, x));
        }
    }
}

void miniTree () {
    E = 0;
    memset(first, -1, sizeof(first));
    for (int i = 0; i < N; i++) F[i] = i;

    int n = N;
    sort(S.begin(), S.end());
    for (int i = 0; i < S.size(); i++) {
        int u = S[i].u, v = S[i].v;
        if (find(u) != find(v)) {
            F[find(u)] = find(v);
            addEdge(u, v, n);
            addEdge(v, u, n);

            set(u, v, inf);
            W[n--] = S[i].w;

            if (n == 1) break;
        }
    }
}

int dfs (int u, int fa, int rt, int id, int w) {
//  printf("rt=%d fa=%d u=%d %d %d\n", rt, fa, u, id, w);
    for (int i = first[u]; i + 1; i = jump[i]) {
        int v = linker[i];
        if (v == fa) continue;
        w = min(w, dfs(v, u, rt, idx[i], get(rt, v)));
    }
    T[id] = min(T[id], w);
    return w;
}

int main () {
    int cas;
    scanf("%d", &cas);
    while (cas--) {
        init();
        miniTree();
        for (int i = 1; i <= N; i++) T[i] = inf;
        for (int i = 0; i < N; i++) dfs(i, i, i, 1, inf);

        int ans = 0;
        for (int i = 2; i <= N; i++) {
            if (T[i] != W[i]) ans++;
        }
        printf("%d\n", ans);
    }
    return 0;
}