HDU 2586 How far away ？ (LCA multiplication method)

The question ： Give a spanning tree with edge weight ,q Time to ask , seek u、v The length of the path between .

Answer key ：lca
Multiplication method .

#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#include<queue>
#include<stack>
#include<cmath>
#include<vector>
#include<fstream>
#include<set>
#include<map>
#include<sstream>
#include<iomanip>
#define ll long long
#define pii pair<int, int>
using namespace std;
const int MAXN = 4e4 + 5;
int t, n, q;
vector<int> v[MAXN], w[MAXN];
int fa[MAXN][31], cost[MAXN][31], dep[MAXN];
void init() {
    
    for (int i = 1; i <= n; i++) v[i].clear(), w[i].clear();
    /*memset(fa, 0, sizeof(fa)); memset(cost, 0, sizeof(cost)); memset(dep, 0, sizeof(dep));*/
}
void dfs(int root, int fno) {
       //1 0
    fa[root][0] = fno;
    dep[root] = dep[fa[root][0]] + 1;
    for (int i = 1; i < 31; ++i) {
    
        fa[root][i] = fa[fa[root][i - 1]][i - 1];
        cost[root][i] = cost[fa[root][i - 1]][i - 1] + cost[root][i - 1];
    }
    int sz = v[root].size();
    for (int i = 0; i < sz; ++i) {
    
        if (v[root][i] == fno) continue;
        cost[v[root][i]][0] = w[root][i];
        dfs(v[root][i], root);
    }
}
int lca(int x, int y) {
    
    if (dep[x] > dep[y]) swap(x, y);
    int tmp = dep[y] - dep[x], ans = 0;
    for (int j = 0; tmp; ++j, tmp >>= 1)
        if (tmp & 1) ans += cost[y][j], y = fa[y][j];
    if (y == x) return ans;
    for (int j = 30; j >= 0 && y != x; --j) {
    
        if (fa[x][j] != fa[y][j]) {
    
            ans += cost[x][j] + cost[y][j];
            x = fa[x][j];
            y = fa[y][j];
        }
    }
    ans += cost[x][0] + cost[y][0];
    return ans;
}
int main() {
    
	scanf("%d", &t);
    int x, y, k;
	while (t--) {
    
        init();
		scanf("%d%d", &n, &q);
		for (int i = 1; i < n; i++) {
    
            scanf("%d%d%d", &x, &y, &k);
            v[x].push_back(y);
            v[y].push_back(x);
            w[x].push_back(k);
            w[y].push_back(k);
		}
        dfs(1, 0);
        while (q--) {
    
            scanf("%d%d", &x, &y);
            printf("%d\n", lca(x, y));
        }
	}
	return 0;
}