Aprendi LCA!

Problems/SphereOJ/lcasq.cpp

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+#include <bits/stdc++.h>
+
+using namespace std;
+
+vector<vector<int>> bl;
+vector<vector<int>> adj;
+vector<int> depth;
+int bllog2;
+
+void ddfs(int o){
+	for(int nv : adj[o]){
+		depth[nv] = depth[o]+1;
+		ddfs(nv);
+	}
+}
+
+int lca(int a, int b){
+	if(depth[a] < depth[b]) swap(a,b);
+
+	int k = depth[a] - depth[b];
+	for(int p = bllog2; p >= 0; p--){
+		if(k & (1 << p)){
+			a = bl[a][p];
+		}
+	}
+
+	if(a == b) return a;
+
+	for(int p = bllog2; p >= 0; p--){
+		if(bl[a][p] != bl[b][p]){
+			a = bl[a][p], b = bl[b][p];
+		}
+	}
+
+	return bl[a][0];
+}
+
+int main(){
+	ios::sync_with_stdio(false);
+
+	int n,q, m, x,y;
+	cin >> n;
+
+	adj.assign(n, vector<int>());
+	bllog2 = 32 - __builtin_clz(n);
+	bl.assign(n, vector<int>(bllog2+1));	
+	bl[0][0] = 0;
+	for(int i = 0; i < n; i++){
+		cin >> m;
+		while(m--){
+			cin >> x;
+			bl[x][0] = i;
+			adj[i].push_back(x);
+		}
+	}
+
+	for(int p = 1; p <= bllog2; p++){
+		for(int i = 0; i < n; i++){
+			x = bl[i][p-1];
+			bl[i][p] = bl[x][p-1];
+		}
+	}
+	depth.resize(n);
+	depth[0] = 0;
+	ddfs(0);
+
+	cin >> q;
+	while(q--){
+		cin >> x >> y;
+		cout << lca(x,y) << '\n';
+	}
+
+	return 0;
+}