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LCA.cpp
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LCA.cpp
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#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
vector<vector<int>> edge;
vector<int> level;
vector<vector<int>> sparse;
void solve_level_sparse(int n, int root) {
queue<int> q;
sparse[root][0] = root; level[root] = 0;
q.push(root);
while (!q.empty()) {
int s = q.front(); q.pop();
for (int i = 0; i < edge[s].size(); i++) {
int e = edge[s][i];
if (level[e] == -1) {
sparse[e][0] = s;
level[e] = level[s]+1;
q.push(e);
}
}
}
for (int j = 0; j < 31; j++) {
for (int i = 0; i < n; i++) {
sparse[i][j+1] = sparse[sparse[i][j]][j];
}
}
}
int lca(int a, int b) {
if (level[a] < level[b]) swap(a, b);
int diff = level[a]-level[b];
for(int i = 0; diff; i++){
if (diff&1) a = sparse[a][i];
diff >>= 1;
}
if (a == b) return a;
for (int i = 31; i >= 0; i--){
if (sparse[a][i] != sparse[b][i]) a = sparse[a][i], b = sparse[b][i];
}
return sparse[a][0];
}
int main() {
ios_base :: sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
int n; cin >> n;
edge.resize(n, vector<int>());
level.resize(n, -1);
sparse.resize(n, vector<int>(32, -1));
for (int i = 0; i < n-1; i++) {
int a, b, w; cin >> a >> b;
a--; b--;
edge[a].push_back(b);
edge[b].push_back(a);
}
solve_level_sparse(n, 0);
int m; cin >> m;
for (int i = 0; i < m; i++) {
int a, b; cin >> a >> b;
a--; b--;
cout << lca(a, b)+1 << '\n';
}
}