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LCA.cpp
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LCA.cpp
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#include<bits/stdc++.h>
using namespace std;
const int N = 3e5 + 9, LG = 18;
vector<int> g[N];
int par[N][LG + 1], dep[N], sz[N];
void dfs(int u, int p = 0) {
par[u][0] = p;
dep[u] = dep[p] + 1;
sz[u] = 1;
for (int i = 1; i <= LG; i++) par[u][i] = par[par[u][i - 1]][i - 1];
for (auto v: g[u]) if (v != p) {
dfs(v, u);
sz[u] += sz[v];
}
}
int lca(int u, int v) {
if (dep[u] < dep[v]) swap(u, v);
for (int k = LG; k >= 0; k--) if (dep[par[u][k]] >= dep[v]) u = par[u][k];
if (u == v) return u;
for (int k = LG; k >= 0; k--) if (par[u][k] != par[v][k]) u = par[u][k], v = par[v][k];
return par[u][0];
}
int kth(int u, int k) {
assert(k >= 0);
for (int i = 0; i <= LG; i++) if (k & (1 << i)) u = par[u][i];
return u;
}
int dist(int u, int v) {
int l = lca(u, v);
return dep[u] + dep[v] - (dep[l] << 1);
}
//kth node from u to v, 0th node is u
int go(int u, int v, int k) {
int l = lca(u, v);
int d = dep[u] + dep[v] - (dep[l] << 1);
assert(k <= d);
if (dep[l] + k <= dep[u]) return kth(u, k);
k -= dep[u] - dep[l];
return kth(v, dep[v] - dep[l] - k);
}
int32_t main() {
int n; cin >> n;
for (int i = 1; i < n; i++) {
int u, v; cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
dfs(1);
int q; cin >> q;
while (q--) {
int u, v; cin >> u >> v;
cout << dist(u, v) << '\n';
}
return 0;
}