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OI-codes/Luogu/P3379/P3379.cpp

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#include <bits/stdc++.h>
using namespace std;
int n, m, s, root, a, b, dep[500005], fa[500005][20];
vector<int> tr[500005];
void bfs(int root) {
memset(dep, 0x3f, sizeof(dep));
dep[0] = 0;
dep[root] = 1;
queue<int> q;
q.push(root);
while (!q.empty()) {
int t = q.front();
q.pop();
for (int i : tr[t]) {
if (dep[i] > dep[t] + 1) {
dep[i] = dep[t] + 1;
q.push(i);
fa[i][0] = t;
for (int k = 1; k <= 19; k++) {
fa[i][k] = fa[fa[i][k - 1]][k - 1];
}
}
}
}
}
int lca(int a, int b) {
if (dep[a] < dep[b]) swap(a, b);
for (int k = 19; k >= 0; k--) {
if (dep[fa[a][k]] >= dep[b]) {
a = fa[a][k];
}
}
if (a == b) return a;
for (int k = 19; k >= 0; k--) {
if (fa[a][k] != fa[b][k]) {
a = fa[a][k];
b = fa[b][k];
}
}
return fa[a][0];
}
int main() {
cin >> n >> m >> root;
for (int i = 1; i < n; i++) {
cin >> a >> b;
tr[a].push_back(b);
tr[b].push_back(a);
}
bfs(root);
for (int i = 0; i < m; i++) {
cin >> a >> b;
cout << lca(a, b) << endl;
}
return 0;
}