P2245 星际导航

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模版题

代码 
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#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pii pair<int, int>
#define mk make_pair
const int N = 1e6 + 10;
const int mod = 1e9 + 7;

struct Edge
{
    int u, v, w;

    bool operator<(const Edge &b) const
    {
        return w < b.w;
    }
};
namespace KruskalTree
{
    int fa[N];
    int f[N][20];
    int dep[N], a[N];
    vector<int> g[N];
    int find(int x)
    {
        return fa[x] == x ? x : fa[x] = find(fa[x]);
    }
    void addedge(int u, int v)
    {
        g[u].push_back(v);
        g[v].push_back(u);
    }
    void dfs1(int x, int ff)
    {
        f[x][0] = ff;
        dep[x] = dep[ff] + 1;
        for (int i = 1; i <= 18; i++)
            f[x][i] = f[f[x][i - 1]][i - 1];
        for (int to : g[x])
        {
            if (to == ff)
                continue;
            dfs1(to, x);
        }
    }
    int lca(int u, int v)
    {
        if (dep[u] < dep[v])
            swap(u, v);
        for (int i = 18; i >= 0; i--)
        {
            if (dep[f[u][i]] >= dep[v])
                u = f[u][i];
        }
        if (u == v)
            return u;
        for (int i = 18; i >= 0; i--)
        {
            if (f[u][i] != f[v][i])
                u = f[u][i], v = f[v][i];
        }

        return f[u][0];
    }
    void init(int n, vector<Edge> e)
    {
        for (int i = 0; i <= n << 1; i++)
            fa[i] = i;
        int cnt = n;
        sort(e.begin(), e.end());
        for (Edge now : e)
        {
            int u = now.u, v = now.v, w = now.w;
            int x = find(u), y = find(v);
            if (x == y)
                continue;
            cnt++;
            fa[x] = fa[y] = cnt;
            addedge(x, cnt);
            addedge(y, cnt);
            a[cnt] = w;
        }

        for (int i = cnt; i >= 1; i--)
            if (!dep[i])
            {

                dfs1(i, 0);
            }
    }

} // namespace KruskalTree

int read()
{
    int x = 0, f = 1;
    char c = getchar();
    while (c < '0' || c > '9')
    {
        if (c == '-')
            f = -1;
        c = getchar();
    }
    while (c >= '0' && c <= '9')
        x = (x << 1) + (x << 3) + c - '0', c = getchar();
    return x * f;
}

using namespace KruskalTree;
int main()
{
    int n = read(), m = read();
    vector<Edge> e;
    for (int i = 1; i <= m; i++)
    {
        int u = read(), v = read(), w = read();
        e.push_back((Edge){u, v, w});
    }
    init(n, e);
    int q = read();
    for (int i = 1; i <= q; i++)
    {
        int u = read(), v = read();
        if (find(u) != find(v))
            puts("impossible");
        else
            printf("%d\n", a[lca(u, v)]);
    }
}