板的不能再板，链剖+线段树或者是LCT随便维护。
感觉唯一要注意的是跳链的时候要对$x$向上跳和$y$向上跳的情况分开讨论，而不能直接$swap$，因为只有两段接触的端点才能相互合并，而且每一次向上跳的线段要放在已经合并完成之后的左端。
最后输出答案的时候要注意这时候$x$和$y$合并好的树链上其实左端点都是在上面的，而$x$和$y$其实是左端点相接触的，所以最后合并的时候$x.lmax + y.lmax$可能成为最优答案而不是$x.rmax + y.lmax$。
一开始线段树写手残了……
时间复杂度$O(nlog^2n)$。
Code:
#include <cstdio> #include <cstring> using namespace std; const int N = 1e5 + 5; int n, qn, tot = 0, head[N], a[N], w[N]; int dfsc = 0, id[N], siz[N], dep[N], son[N], fa[N], top[N]; struct Edge { int to, nxt; } e[N << 1]; inline void add(int from, int to) { e[++tot].to = to; e[tot].nxt = head[from]; head[from] = tot; } inline void read(int &X) { X = 0; char ch = 0; int op = 1; for(; ch > '9' || ch < '0'; ch = getchar()) if(ch == '-') op = -1; for(; ch >= '0' && ch <= '9'; ch = getchar()) X = (X << 3) + (X << 1) + ch - 48; X *= op; } inline void swap(int &x, int &y) { int t = x; x = y; y = t; } inline int max(int x, int y) { return x > y ? x : y; } inline int max(int x, int y, int z) { return max(max(x, y), z); } void dfs1(int x, int fat, int depth) { dep[x] = depth, fa[x] = fat, siz[x] = 1; int maxson = -1; for(int i = head[x]; i; i = e[i].nxt) { int y = e[i].to; if(y == fat) continue; dfs1(y, x, depth + 1); siz[x] += siz[y]; if(siz[y] > maxson) maxson = siz[y], son[x] = y; } } void dfs2(int x, int topf) { top[x] = topf, w[id[x] = ++dfsc] = a[x]; if(!son[x]) return; dfs2(son[x], topf); for(int i = head[x]; i; i = e[i].nxt) { int y = e[i].to; if(y == fa[x] || y == son[x]) continue; dfs2(y, y); } } struct Node { int lmax, rmax, sum, res, val; bool tag; inline void init() { lmax = rmax = sum = res = val = 0; tag = 0; } }; inline Node merge(Node x, Node y) { Node ans; ans.init(); ans.sum = x.sum + y.sum; ans.lmax = max(x.lmax, x.sum + y.lmax); ans.rmax = max(y.rmax, y.sum + x.rmax); ans.res = max(x.res, y.res, x.rmax + y.lmax); return ans; } namespace SegT { Node s[N << 2]; #define lc p << 1 #define rc p << 1 | 1 #define mid ((l + r) >> 1) #define lmax(p) s[p].lmax #define rmax(p) s[p].rmax #define sum(p) s[p].sum #define res(p) s[p].res #define tag(p) s[p].tag #define val(p) s[p].val inline void up(int p) { if(!p) return; lmax(p) = max(lmax(lc), sum(lc) + lmax(rc)); rmax(p) = max(rmax(rc), sum(rc) + rmax(lc)); sum(p) = sum(lc) + sum(rc); res(p) = max(res(lc), res(rc), rmax(lc) + lmax(rc)); } inline void down(int p, int l, int r) { if(!tag(p)) return; sum(lc) = (mid - l + 1) * val(p), sum(rc) = (r - mid) * val(p); res(lc) = lmax(lc) = rmax(lc) = max(0, val(p) * (mid - l + 1)); res(rc) = lmax(rc) = rmax(rc) = max(0, val(p) * (r - mid)); val(lc) = val(rc) = val(p); tag(lc) = tag(rc) = 1, tag(p) = 0; } void build(int p, int l, int r) { s[p].init(); if(l == r) { sum(p) = w[l]; lmax(p) = rmax(p) = res(p) = max(0, w[l]); return; } build(lc, l, mid); build(rc, mid + 1, r); up(p); } void modify(int p, int l, int r, int x, int y, int v) { if(x <= l && y >= r) { sum(p) = (r - l + 1) * v; lmax(p) = rmax(p) = res(p) = max(0, sum(p)); tag(p) = 1, val(p) = v; return; } down(p, l, r); if(x <= mid) modify(lc, l, mid, x, y, v); if(y > mid) modify(rc, mid + 1, r, x, y, v); up(p); } /* Node query(int p, int l, int r, int x, int y) { if(x <= l && y >= r) return s[p]; down(p, l, r); if(y <= mid) return query(lc, l, mid, x, y); if(x > mid) return query(rc, mid + 1, r, x, y); Node res, ln = query(lc, l, mid, x, mid), rn = query(rc, mid + 1, y, mid + 1, y); res.init(); res.sum = ln.sum + rn.sum; res.lmax = max(ln.lmax, ln.sum + rn.lmax); res.rmax = max(rn.rmax, rn.sum + ln.rmax); res.res = max(ln.res, rn.res, ln.rmax + rn.lmax); return res; } */ Node query(int p, int l, int r, int x, int y) { if(x <= l && y >= r) return s[p]; down(p, l, r); Node ln, rn, ans; ln.init(), rn.init(), ans.init(); if(x <= mid) ln = query(lc, l, mid, x, y); if(y > mid) rn = query(rc, mid + 1, r, x, y); ans = merge(ln, rn); return ans; } } using namespace SegT; inline void mTree(int x, int y, int v) { for(; top[x] != top[y]; ) { if(dep[top[x]] < dep[top[y]]) swap(x, y); modify(1, 1, n, id[top[x]], id[x], v); x = fa[top[x]]; } if(dep[x] > dep[y]) swap(x, y); modify(1, 1, n, id[x], id[y], v); } inline void qTree(int x, int y) { Node ln, rn; ln.init(), rn.init(); for(; top[x] != top[y]; ) { if(dep[top[x]] > dep[top[y]]) ln = merge(query(1, 1, n, id[top[x]], id[x]), ln), x = fa[top[x]]; else rn = merge(query(1, 1, n, id[top[y]], id[y]), rn), y = fa[top[y]]; } if(dep[x] > dep[y]) ln = merge(query(1, 1, n, id[y], id[x]), ln); else rn = merge(query(1, 1, n, id[x], id[y]), rn); printf("%d ", max(ln.res, rn.res, ln.lmax + rn.lmax)); } int main() { read(n); for(int i = 1; i <= n; i++) read(a[i]); for(int x, y, i = 1; i < n; i++) { read(x), read(y); add(x, y), add(y, x); } dfs1(1, 0, 1), dfs2(1, 1); build(1, 1, n); read(qn); for(int op, x, y; qn--; ) { read(op), read(x), read(y); if(op == 2) { int v; read(v); mTree(x, y, v); } else qTree(x, y); } return 0; }