题目链接：

1.把边权转化到点权：选取连接这条边的两个点中较深的一个。

2.查询点到点之间的边权时，要从seg[x]+1 到 seg[y]，因为seg[x]其实连接的是上面一条边的边权。

#include 
    
     #include 
     
      #include 
      
       #include 
       
        #define ll long long#define lson left, mid, rt<<1#define rson mid + 1, right, rt<<1|1using namespace std;const ll maxn = 300000 + 10;char opt[9];ll n, m, root, res;ll top[maxn], rev[maxn], seg[maxn], son[maxn];ll fa[maxn], size[maxn], deep[maxn];ll u[maxn], v[maxn], w[maxn];ll tree[maxn<<2], lazy[maxn<<2], num, node[maxn];struct edge{    ll next, from, to, len;}e[maxn<<2];ll head[maxn], cnt;void add(ll u, ll v, ll w){    e[++cnt].from = u;    e[cnt].len = w;    e[cnt].next = head[u];    e[cnt].to = v;    head[u] = cnt;}//-----segment_tree-----void PushUP(ll rt){    tree[rt] = max(tree[rt<<1], tree[rt<<1|1]);}void build(ll left, ll right, ll rt){    if(left == right)    {        tree[rt] = rev[left];        return;    }    ll mid = (left + right)>>1;    build(lson);    build(rson);    PushUP(rt);}void PushDOWN(ll rt){    lazy[rt<<1] = lazy[rt];    lazy[rt<<1|1] = lazy[rt];    tree[rt<<1] = lazy[rt];    tree[rt<<1|1] = lazy[rt];    lazy[rt] = 0;}void update(ll l, ll r, ll add, ll left, ll right, ll rt){    if(l <= left && r >= right)    {        tree[rt] = add;        lazy[rt] = add;        return;    }    ll mid = (left + right)>>1;    if(lazy[rt]) PushDOWN(rt);    if(l <= mid) update(l, r, add, lson);    if(r > mid) update(l, r, add, rson);    PushUP(rt);}ll query(ll l, ll r, ll left, ll right, ll rt){    ll res = -0x7fffffff;    if(l <= left && r >= right)    {        return tree[rt];    }    ll mid = (left + right)>>1;    if(lazy[rt]) PushDOWN(rt);    if(l <= mid) res = max(res, query(l, r, lson));    if(r > mid) res = max(res, query(l, r, rson));    return res;}//----------------------void dfs1(ll u, ll f, ll d){    ll maxson = -1;    size[u] = 1;    deep[u] = d;    fa[u] = f;    for(ll i = head[u]; i != -1; i = e[i].next)    {        ll v = e[i].to;        if(f != v)        {            dfs1(v, u, d+1);            size[u] += size[v];            if(size[v] > maxson) son[u] = v, maxson = size[v];        }    }}void dfs2(ll u, ll t){    seg[u] = ++num;    rev[num] = node[u];    top[u] = t;    if(!son[u]) return;    dfs2(son[u], t);    for(ll i = head[u]; i != -1; i = e[i].next)    {        ll v = e[i].to;        if(v == son[u] || v == fa[u]) continue;        dfs2(v,v);    }}void updRange(ll x, ll y, ll k){    while(top[x] != top[y])    {        if(deep[top[x]] < deep[top[y]]) swap(x, y);        update(seg[top[x]], seg[x], k, 1, n, 1);        x = fa[top[x]];    }    if(deep[x] > deep[y]) swap(x, y);    update(seg[x], seg[y], k, 1, n, 1);}ll qRange(ll x, ll y){    ll ans = 0;    while(top[x] != top[y])    {        if(deep[top[x]] < deep[top[y]]) swap(x, y);        res = 0;        res = query(seg[top[x]], seg[x], 1, n, 1);        ans = max(res, ans);        x = fa[top[x]];    }    if(deep[x] > deep[y]) swap(x, y);    res = 0;    res = query(seg[x]+1, seg[y], 1, n, 1);    ans = max(res, ans);    return ans;}int main(){    memset(head, -1, sizeof(head));    scanf("%lld",&n);    root = 1;    for(ll i = 1; i < n; i++)    {        scanf("%lld%lld%lld",&u[i],&v[i],&w[i]);        add(u[i],v[i],w[i]); add(v[i],u[i],w[i]);    }    dfs1(root, 0, 1);    //for(int i = 1; i <= n; i++) cout<
        
         <<" ";    for(ll i = 1; i < n; i++)    {        if(deep[u[i]] < deep[v[i]]) node[v[i]] = w[i];        else node[u[i]] = w[i];    }    //for(int i = 1; i <= n; i++) cout<
         
          <<" "; dfs2(root, root); build(1, n, 1); while(cin>>opt && opt[0] != 'D') { ll x, y, z; if(opt[0] == 'C') { scanf("%lld%lld",&x,&y); if(deep[u[x]] > deep[v[x]]) updRange(u[x], u[x], y); else updRange(v[x], v[x], y); } else { scanf("%lld%lld",&x,&y); if(x == y) { printf("0\n"); continue; } printf("%lld\n",qRange(x,y)); } }}