Luogu 2912 [USACO08OCT]牧场散步Pasture Walking

快乐树剖

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #define rd read()
 #define lson nd << 1
 #define rson nd << 1 | 1
 using namespace std;

 const int N = 1e4;

 int dep[N], top[N], f[N], son[N], size[N], id[N];
 int q, n, head[N], tot, cnt, A[N], a[N];
 int sum[N << ];

 struct edge {
     int nxt, to, val;
 }e[N << ];

 int read() {
     int X = , p = ; char c = getchar();
     for(; c > '' || c < ''; c = getchar()) if(c == '-') p = -;
     for(; c >= '' && c <= ''; c = getchar()) X = X *  + c - '';
     return X * p;
 }

 void added(int u, int v, int val) {
     e[++tot].to = v;
     e[tot].nxt = head[u];
     e[tot].val = val;
     head[u] = tot;
 }

 void add(int u, int v, int val) {
     added(u, v, val);
     added(v, u, val);
 }

 void dfs1(int u) {
     size[u] = ;
     for(int i = head[u]; i; i = e[i].nxt) {
         int nt = e[i].to;
         if(nt == f[u]) continue;
         f[nt] = u; dep[nt] = dep[u] + ;
         a[nt] = e[i].val;
         dfs1(nt);
         size[u] += size[nt];
         if(size[nt] > size[son[u]]) son[u] = nt;
     }
 }

 void dfs2(int u) {
     id[u] = ++cnt;
     A[cnt] = a[u];
     if(!son[u]) return;
     top[son[u]] = top[u];
     dfs2(son[u]);
     for(int i = head[u]; i; i = e[i].nxt) {
         int nt = e[i].to;
         if(nt == f[u] || nt == son[u]) continue;
         top[nt] = nt;
         dfs2(nt);
     }
 }

 void update(int nd) {
     sum[nd] = sum[lson] + sum[rson];
 }

 void build(int l, int r, int nd) {
     if(l == r) { sum[nd] = A[l]; return;}
     int mid = (l + r) >> ;
     build(l, mid, lson);
     build(mid + , r, rson);
     update(nd);
 }

 int trie_sum(int L, int R, int l, int r, int nd) {
     if(L <= l && r <= R) return sum[nd];
     int tmp = , mid = (l + r) >> ;
     if(L <= mid) tmp += trie_sum(L, R, l, mid, lson);
     if(mid < R) tmp += trie_sum(L, R, mid + , r, rson);
     return tmp;
 }

 void sw(int &x, int &y) {
     x ^= y, y ^= x, x ^= y;
 }

 int query(int x, int y) {
     int ans = , tmp;
     for(; top[x] != top[y];) {
         if(dep[top[x]] < dep[top[y]]) swap(x, y);
         tmp = trie_sum(id[top[x]], id[x], , n, );
         ans += tmp;
         x = f[top[x]];
     }
     if(dep[x] < dep[y]) swap(x, y);
     tmp = trie_sum(id[y], id[x], , n, );
     ans += tmp - a[y];
     return ans;
 }

 int main()
 {
     n = rd; q = rd;
     for(int i = ; i < n; ++i) {
         int x = rd, y = rd, z = rd;
         add(x, y, z);
     }
     dfs1(); dfs2();
     build(, n, );
     for(; q; q--) {
         int x = rd, y = rd;
         printf("%d\n", query(x, y));
     }
 }