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