题目链接：https://www.luogu.org/problemnew/show/P4114
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 <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define ll long long
#define lson left, mid, rt<<1
#define rson mid + 1, right, rt<<1|1
using 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<<deep[i]<<" ";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<<node[i]<<" ";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));}}
}