题目链接：https://www.luogu.com.cn/problem/P1967
参考资料：https://www.cnblogs.com/qfxlzhe/p/15153705.html
示例程序：
#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e4 + 5, maxm = 5e4 + 5;int n, m, q, f[maxn];
int fa[maxn][16], tr[maxn], idx, dep[maxn];
vector<int> g[maxn];void init() {for (int i = 1; i < 2*n; i++)f[i] = fa[i][0] = i;idx = n;
}int find(int x) {return x == f[x] ? x : f[x] = find(f[x]);
}struct Edge {int u, v, w;bool operator < (const Edge &b) const {return w > b.w;}
} e[maxm];void dfs(int u, int d) {dep[u] = d;for (auto v : g[u])dfs(v, d+1);
}void kruskal_build_tree() {init();sort(e, e+m);for (int i = 0; i < m; i++) {int u = e[i].u, v = e[i].v, w = e[i].w;int x = find(u), y = find(v);if (x != y) {int z = ++idx;tr[z] = w;fa[z][0] = z;g[z].push_back(x);g[z].push_back(y);fa[x][0] = fa[y][0] = z;f[x] = f[y] = z;}}for (int i = 1; i <= idx; i++)if (fa[i][0] == i)dfs(i, 0);for (int i = 1; i < 16; i++)for (int u = 1; u <= idx; u++)fa[u][i] = fa[ fa[u][i-1] ][i-1];
}int lca(int x, int y) {if (find(x) != find(y))return -1;if (dep[x] < dep[y])swap(x, y);for (int i = 15; i >= 0; i--)if (dep[ fa[x][i] ] >= dep[y])x = fa[x][i];if (x == y)return tr[x];for (int i = 15; i >= 0; i--)if (fa[x][i] != fa[y][i])x = fa[x][i], y = fa[y][i];x = fa[x][0];return tr[x];
}int main() {scanf("%d%d", &n, &m);for (int i = 0; i < m; i++)scanf("%d%d%d", &e[i].u, &e[i].v, &e[i].w);kruskal_build_tree();scanf("%d", &q);while (q--) {int x, y;scanf("%d%d", &x, &y);printf("%d\n", lca(x, y));}return 0;
}
