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| #include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 10;
int n, m, k, T;
vector<int> g[N];
int fa[N][17], depth[N], lg[N];
void init() {
for (int i = 1; i <= n; ++i)
lg[i] = lg[i - 1] + (1 << lg[i - 1] == i);
}
void dfs(int cur, int father) {
fa[cur][0] = father;
depth[cur] = depth[father] + 1;
for (int i = 1; i <= lg[depth[cur]]; ++i) {
fa[cur][i] = fa[fa[cur][i - 1]][i - 1];
}
for (auto i : g[cur]) {
if (i != father)
dfs(i, cur);
}
}
int LCA(int x, int y) {
if (depth[x] < depth[y])
swap(x, y);
while (depth[x] > depth[y])
{
x = fa[x][lg[depth[x] - depth[y]] - 1];
}
if (x == y)
return x;
for (int k = lg[depth[x]] - 1; k >= 0; --k)
{
if (fa[x][k] != fa[y][k]) {
x = fa[x][k], y = fa[y][k];
}
}
return fa[x][0];
}
int root;
int main() {
cin >> n >> m >> root;
init();
n--;
while (n--) {
int x, y;
cin >> x >> y;
g[x].push_back(y);
g[y].push_back(x);
}
dfs(root, 0);
while (m--) {
int x, y;
cin >> x >> y;
int tmp = LCA(x, y);
cout << tmp << '\n';
}
return 0;
}
|