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OI-codes/Luogu/P3379/P3379.cpp

88 lines
1.5 KiB
C++

#include <iostream>
#include <algorithm>
#include <queue>
#include <vector>
using std::cin;
using std::cout;
const char endl = '\n';
const int N = 5e5 + 5;
const int INF = 0x3f3f3f3f;
int n, m, s, dep[N], fa[N][25];
std::vector<int> g[N];
void bfs(int s) {
std::fill_n(dep, N, INF);
std::queue<int> q;
q.emplace(s);
dep[0] = 0;
dep[s] = 1;
while (!q.empty()) {
int u = q.front();
q.pop();
for (int v : g[u]) {
if (dep[v] > dep[u] + 1) {
dep[v] = dep[u] + 1;
q.emplace(v);
fa[v][0] = u;
for (int k = 1; k <= 19; k++) {
fa[v][k] = fa[fa[v][k - 1]][k - 1];
}
}
}
}
}
int lca(int x, int y) {
if (dep[x] < dep[y]) std::swap(x, y);
for (int k = 19; k >= 0; k--) {
if (dep[fa[x][k]] >= dep[y]) {
x = fa[x][k];
}
}
if (x == y) return x;
for (int k = 19; k >= 0; k--) {
if (fa[x][k] != fa[y][k]) {
x = fa[x][k];
y = fa[y][k];
}
}
return fa[x][0];
}
int main() {
std::ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m >> s;
for (int i = 1, u, v; i < n; i++) {
cin >> u >> v;
g[u].emplace_back(v);
g[v].emplace_back(u);
}
bfs(s);
while (m--) {
int a, b;
cin >> a >> b;
cout << lca(a, b) << endl;
}
return 0;
}