求助，想不明白求环内的距离的一些细节

inline void solve(int u,int v,int w){
    //参数w为非树边的边权
    ++ext;
    int pw,pre = w,i = v;
    while(i!=fa[u]){
        sum[i] = pre;
        pre += b[i];
        i = fa[i];
    }
    sum[ext] = sum[u]; //把整个环的边权和存到方点上
    sum[u] = 0;//这里
    i = v;
    while(i!=fa[u]){
        pw = min(sum[i],sum[ext]-sum[i]);
        //找最短路,建树边
        adj[ext].push_back(edge(i,pw));
        adj[i].push_back(edge(ext,pw));
        i = fa[i];
    }
}