给你一棵树，每次询问一个(a,b)，问有多少有路径与a-b没有交集
找lca
#include <bits/stdc++.h> using namespace std; #define rep(i, j, k) for (int i = int(j); i <= int(k); ++ i) #define dwn(i, j, k) for (int i = int(j); i >= int(k); -- i) typedef long long LL; typedef pair<int, int> P; const int N = 1e5 + 7; vector<int> g[N]; int dep[N], fa[N][20], sz[N]; LL f1[N], f2[N]; int n, q; LL calc(LL x) { return x * (x + 1) / 2LL; } void dfs(int u, int f) { dep[u] = dep[f] + 1; fa[u][0] = f; sz[u] = 1; for (int v: g[u]) if (v != f) { dfs(v, u); sz[u] += sz[v]; } } void dfs2(int u, int f) { for (int v: g[u]) if (v != f) { dfs2(v, u); f1[u] += calc(sz[v]); } } void dfs3(int u, int f) { if (u != 1) f2[u] = f2[f] + f1[f] - calc(sz[u]); for (int v: g[u]) if (v != f) dfs3(v, u); } int lca(int u, int v) { if (dep[u] < dep[v]) swap(u, v); for (int i = 19; i >= 0; --i) if (dep[u] - (1 << i) >= dep[v]) u = fa[u][i]; if (u == v) return u; for (int i = 19; i >= 0; --i) if (fa[u][i] && fa[u][i] != fa[v][i]) u = fa[u][i], v = fa[v][i]; return fa[u][0]; } int find(int u, int d) { dwn(i, 19, 0) if (d >= (1 << i)) u = fa[u][d]; return u; } LL solve(int x, int y) { if (dep[x] < dep[y]) swap(x, y); int lc = lca(x, y); // dep[x] >= dep[y] LL ret = 0; if (y == lc) { ret = f1[x] + f2[x] - f2[y] + calc(n - sz[y]); } else { ret = f1[x] + f1[y] + calc(n - sz[lc]); ret += f2[x] - f2[lc]; int t1 = find(y, dep[lc] - dep[y] - 1); ret -= calc(sz[t1]); ret += f2[y] - f2[lc]; int t2 = find(x, dep[lc] - dep[x] - 1); ret -= calc(sz[t2]); ret -= f1[lc] - calc(sz[t1]) - calc(sz[t2]); } return ret; } int main() { scanf("%d%d", &n, &q); rep(i, 1, n - 1) { int x, y; scanf("%d%d", &x, &y); g[x].push_back(y); g[y].push_back(x); } dfs(1, 0); rep(j, 1, 19) rep(i, 1, n) fa[i][j] = fa[fa[i][j - 1]][j - 1]; dfs2(1, 0); dfs3(1, 0); // rep(i, 1, n) cout << f2[i] << ' '; cout << '\n'; while (q --) { int x, y; scanf("%d%d", &x, &y); cout << solve(x, y) << '\n'; } } /* 6 2 1 2 3 2 3 4 3 5 6 3 5 4 1 6 */