洛谷 P1967 货车运输

夏令营和济南培训的时候都讲过现在才开始刷
我真的太颓了...
传送门

先放个玄学东西:

思路:

先跑一下kruskal然后，LCA找最小值最大的那条路

代码:

#include <map>
#include <set>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <cstring>
#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <algorithm>

#define N 50005
#define M 100005
#define INF 999999999

using namespace std;
struct Edge1 {
	int x, y, dis;
}edge1[N];
struct Edge2 {
	int to, next, dis;
}edge2[M];
int head[N], fa[N][21], w[N][21], fath[N], cnt, deep[N], n, m;
bool vis[N];

int read() {
	int s = 0, f = 0; char ch = getchar();
	while (!isdigit(ch)) f |= (ch == '-'), ch = getchar();
	while (isdigit(ch)) s = s * 10 + (ch ^ 48), ch = getchar();
	return f ? -s : s;
}

bool CMP(Edge1 x, Edge1 y) {
	return x.dis > y.dis;
}

void add_edge(int from, int to, int dis) {
	edge2[++cnt].next = head[from];
	edge2[cnt].to = to;
	edge2[cnt].dis = dis;
	head[from] = cnt;
}

int find(int x) {
	if (fath[x] != x) fath[x] = find(fath[x]);
	return fath[x];
}

void kruskal() {
	sort(edge1 + 1, edge1 + m + 1, CMP);
	for (int i = 1; i <= n; i++) fath[i] = i; 
	for (int i = 1; i <= m; i++)
		if (find(edge1[i].x) != find(edge1[i].y)) {
			fath[find(edge1[i].x)] = find(edge1[i].y);
			add_edge(edge1[i].x, edge1[i].y, edge1[i].dis);
			add_edge(edge1[i].y, edge1[i].x, edge1[i].dis);
		}
}

void dfs(int node) {
	vis[node] = true;
	for (int i = head[node]; i; i = edge2[i].next) {
		int to = edge2[i].to;
		if (vis[to]) continue;
		deep[to] = deep[node] + 1;
		fa[to][0] = node;
		w[to][0] = edge2[i].dis;
		dfs(to);
	}
}

int lca(int x, int y) {
	if (find(x) != find(y)) return -1;
	int ans = INF;
	if (deep[x] > deep[y]) swap(x, y);
	for (int i = 20; i >= 0; i--)
		if (deep[fa[y][i]] >= deep[x]) {
			ans = min(ans, w[y][i]);
			y = fa[y][i];	
		}
	if (x == y) return ans;
	for (int i = 20; i >= 0; i--) 
		if (fa[x][i] != fa[y][i]) {
			ans = min(ans, min(w[x][i], w[y][i]));
			x = fa[x][i];
			y = fa[y][i];
		}
	ans = min(ans, min(w[x][0], w[y][0]));
	return ans;
}

int main() {
	n = read(), m = read();
	for (int i = 1, x, y, dis; i <= m; i++) {
		x = read(), y = read(), dis = read();
		edge1[i].x = x, edge1[i].y = y, edge1[i].dis = dis;
	}
	kruskal();
	for (int i = 1; i <= n; i++) 
		if (!vis[i]) {
			deep[i] = 1;
			dfs(i);
			fa[i][0] = i;
			w[i][0] = INF;
		}
	for (int i = 1; i <= 20; i++)
		for (int j = 1; j <= n; j++) {
			fa[j][i] = fa[fa[j][i - 1]][i - 1];
			w[j][i] = min(w[j][i - 1], w[fa[j][i - 1]][i - 1]);
		}
	int s = read();
	for (int i = 1, x, y; i <= s; i++) {
		x = read(), y = read();
		printf("%d\n", lca(x, y));
	}
	return 0;
}