【BZOJ 1602】 牧场行走

【题目链接】

           https://www.lydsy.com/JudgeOnline/problem.php?id=1602

【算法】

           倍增求LCA

【代码】

             

#include<bits/stdc++.h>
using namespace std;
#define MAXN 1010
#define MAXLOG 20

int i,n,q,u,v,w;
int sum[MAXN],anc[MAXN][MAXLOG],dep[MAXN];
vector< pair<int,int> > e[MAXN];

inline int lca(int u,int v)
{
        int i,t;
        if (dep[u] > dep[v]) swap(u,v);
        t = dep[v] - dep[u];
        for (i = 0; i < MAXLOG; i++)
        {
                if (t & (1 << i))
                        v = anc[v][i];
        }
        if (u == v) return u;
        for (i = MAXLOG - 1; i >= 0; i--)
        {
                if (anc[u][i] != anc[v][i])
                {
                        u = anc[u][i];
                        v = anc[v][i];
                }
        }
        return anc[u][0];
}
inline void dfs(int u)
{
        int i,v,w;
        for (i = 1; i < MAXLOG; i++) anc[u][i] = anc[anc[u][i-1]][i-1];
        for (i = 0; i < e[u].size(); i++)
        {
                v = e[u][i].first;
                w = e[u][i].second;
                if (anc[u][0] != v)
                {
                        dep[v] = dep[u] + 1;
                        sum[v] = sum[u] + w;
                        anc[v][0] = u;
                        dfs(v);
                }
        }
}
int main() 
{
        
        scanf("%d%d",&n,&q);
        for (i = 1; i < n; i++) 
        {
                scanf("%d%d%d",&u,&v,&w);
                e[u].push_back(make_pair(v,w));
                e[v].push_back(make_pair(u,w));    
        } 
        dfs(1);
        while (q--)
        {
                scanf("%d%d",&u,&v);
                printf("%d\n",sum[u]+sum[v]-2*sum[lca(u,v)]);        
        }
        
        return 0;
    
}