73 lines
1.5 KiB
C++
73 lines
1.5 KiB
C++
#include <bits/stdc++.h>
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using namespace std;
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const int N = 500010, M = N << 1;
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int e[M], h[N], idx, w[M], ne[M];
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void add(int a, int b, int c = 0) {
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e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
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}
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int depth[N];
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int f[N][20];
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void bfs(int root) {
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queue<int> q;
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q.push(root);
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depth[root] = 1;
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while (q.size()) {
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int u = q.front();
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q.pop();
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for (int i = h[u]; ~i; i = ne[i]) {
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int v = e[i];
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if (!depth[v]) {
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depth[v] = depth[u] + 1;
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f[v][0] = u;
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q.push(v);
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for (int k = 1; k <= 19; k++) f[v][k] = f[f[v][k - 1]][k - 1];
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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 (depth[a] < depth[b]) swap(a, b);
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for (int k = 19; k >= 0; k--)
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if (depth[f[a][k]] >= depth[b]) a = f[a][k];
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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 (f[a][k] != f[b][k])
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a = f[a][k], b = f[b][k];
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return f[a][0];
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}
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int main() {
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#ifndef ONLINE_JUDGE
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freopen("P3379.in", "r", stdin);
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#endif
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memset(h, -1, sizeof h);
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// Q:为什么上面的数组开20，每次k循环到19？
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// 答： cout << log2(500000) << endl;
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int n, m, s;
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scanf("%d %d %d", &n, &m, &s);
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int a, b;
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for (int i = 1; i < n; i++) {
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scanf("%d %d", &a, &b);
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add(a, b), add(b, a);
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}
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bfs(s);
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while (m--) {
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scanf("%d %d", &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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} |