# 7 solutions

• @ 2021-12-5 21:03:51

倍增法求 LCA 。

``````#include <bits/stdc++.h>

using std::cin;
using std::cout;
using std::endl;

int n, q, u, v, fa[500005][20], dep[500005];
std::vector<int> g[500005];

void bfs(int root) {
memset(dep, 0x3f, sizeof(dep));
std::queue<int> q;
dep[0] = 0;
dep[root] = 1;
q.push(root);
while (!q.empty()) {
int t = q.front();
q.pop();
for (int v : g[t]) {
if (dep[v] > dep[t] + 1) {
dep[v] = dep[t] + 1;
q.push(v);
fa[v][0] = t;
for (int k = 1; k < 20; k++) {
fa[v][k] = fa[fa[v][k - 1]][k - 1];
}
}
}
}
}

int lca(int a, int b) {
if (dep[a] < dep[b]) std::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() {
scanf("%d%d", &n, &q);
for (int i = 1; i < n; i++) {
scanf("%d%d", &u, &v);
g[u].push_back(v);
g[v].push_back(u);
}
bfs(1);
while (q--) {
scanf("%d%d", &u, &v);
printf("%d\n", lca(u, v));
}
return 0;
}
``````

# Information

ID
121
Time
2000ms
Memory
256MiB
Difficulty
6
Tags
# Submissions
369
Accepted
115