Version 1: 倍增
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using p = pair<int, int>;
const int maxn(5e5 + 10);
const int maxm(1e6 + 10);
int ecnt, head[maxn];
int dep[maxn], f[maxn][20];
struct edge {
int to, nxt;
} edges[maxm];
template<typename T = int>
inline const T read()
{
T x = 0, f = 1;
char ch = getchar();
while (ch < '0' || ch > '9') {
if (ch == '-') f = -1;
ch = getchar();
}
while (ch >= '0' && ch <= '9') {
x = (x << 3) + (x << 1) + ch - '0';
ch = getchar();
}
return x * f;
}
template<typename T>
inline void write(T x, bool ln)
{
if (x < 0) {
putchar('-');
x = -x;
}
if (x > 9) write(x / 10, false);
putchar(x % 10 + '0');
if (ln) putchar(10);
}
void addEdge(int u, int v)
{
edges[ecnt].to = v;
edges[ecnt].nxt = head[u];
head[u] = ecnt++;
}
void bfs(int root)
{
queue<int> q;
q.push(root);
dep[root] = 1;
while (not q.empty()) {
int u = q.front();
q.pop();
for (int i = head[u]; compl i; i = edges[i].nxt) {
int v = edges[i].to;
if (dep[v]) {
continue;
}
q.push(v);
dep[v] = dep[u] + 1;
f[v][0] = u;
for (int j = 1; j < 20; ++j) {
f[v][j] = f[f[v][j - 1]][j - 1];
}
}
}
}
int lca(int u, int v)
{
if (dep[u] < dep[v]) {
swap(u, v);
}
for (int i = 19; i >= 0; --i) {
if (dep[f[u][i]] >= dep[v]) {
u = f[u][i];
}
}
if (u == v) {
return u;
}
for (int i = 19; i >= 0; --i) {
if (f[u][i] not_eq f[v][i]) {
u = f[u][i];
v = f[v][i];
}
}
return f[u][0];
}
int main()
{
#ifdef ONLINE_JUDGE
#else
freopen("input.txt", "r", stdin);
#endif
ios::sync_with_stdio(false);
memset(head, -1, sizeof head);
int n = read(), m = read(), s = read();
for (int i = 0; i < n - 1; ++i) {
int u = read(), v = read();
addEdge(u, v);
addEdge(v, u);
}
bfs(s);
while (m--) {
int u = read(), v = read();
write(lca(u, v), true);
}
return 0;
}
Version 2: 离线 Tarjan
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using p = pair<int, int>;
const int maxn(5e5 + 10);
const int maxm(1e6 + 10);
int ecnt, qcnt;
bool vis[maxn];
int ehead[maxn], qhead[maxm];
int pre[maxn], ans[maxm];
struct edge {
int to, nxt;
} edges[maxm];
struct que {
int id, to, nxt;
} ques[maxm];
template<typename T = int>
inline const T read()
{
T x = 0, f = 1;
char ch = getchar();
while (ch < '0' || ch > '9') {
if (ch == '-') f = -1;
ch = getchar();
}
while (ch >= '0' && ch <= '9') {
x = (x << 3) + (x << 1) + ch - '0';
ch = getchar();
}
return x * f;
}
template<typename T>
inline void write(T x, bool ln)
{
if (x < 0) {
putchar('-');
x = -x;
}
if (x > 9) write(x / 10, false);
putchar(x % 10 + '0');
if (ln) putchar(10);
}
void addEdge(int u, int v)
{
edges[ecnt].to = v;
edges[ecnt].nxt = ehead[u];
ehead[u] = ecnt++;
}
void addQue(int id, int u, int v)
{
ques[qcnt].id = id;
ques[qcnt].to = v;
ques[qcnt].nxt = qhead[u];
qhead[u] = qcnt++;
}
int Find(int x)
{
return pre[x] == x ? x : pre[x] = Find(pre[x]);
}
void Union(int u, int v)
{
pre[Find(v)] = Find(u);
}
void dfs(int cur, int pre)
{
vis[cur] = true;
for (int i = ehead[cur]; compl i; i = edges[i].nxt) {
int nxt = edges[i].to;
if (nxt not_eq pre) {
dfs(nxt, cur);
}
}
for (int i = qhead[cur]; compl i; i = ques[i].nxt) {
int id = ques[i].id, nxt = ques[i].to;
if (vis[nxt]) {
ans[id] = Find(nxt);
}
}
Union(pre, cur);
}
void tarjan(int tot, int root)
{
for (int i = 1; i <= tot; ++i) {
pre[i] = i;
}
dfs(root, 0);
}
int main()
{
#ifdef ONLINE_JUDGE
#else
freopen("input.txt", "r", stdin);
#endif
ios::sync_with_stdio(false);
memset(ehead, -1, sizeof ehead);
memset(qhead, -1, sizeof qhead);
int n = read(), m = read(), s = read();
for (int i = 0; i < n - 1; ++i) {
int u = read(), v = read();
addEdge(u, v);
addEdge(v, u);
}
for (int i = 0; i < m; ++i) {
int u = read(), v = read();
addQue(i, u, v);
addQue(i, v, u);
}
tarjan(n, s);
for (int i = 0; i < m; ++i) {
write(ans[i], true);
}
return 0;
}