backup

graph/lca.cc

@@ -0,0 +1,48 @@
+struct LCA {
+    vector<int> depth;
+    vector<vector<int>> pa;
+    LCA(const vector<vector<int>>& g, int root = 1) {
+        int n = g.size() - 1;
+        int m = 32 - __builtin_clz(n);
+        depth.resize(n + 1);
+        pa.resize(n + 1, vector<int>(m, -1));
+        function<void(int, int)> dfs = [&](int x, int fa) {
+            pa[x][0] = fa;
+            for (int y: g[x]) {
+                if (y != fa) {
+                    depth[y] = depth[x] + 1;
+                    dfs(y, x);
+                }
+            }
+        };
+        dfs(root, 0);
+
+        for (int i = 0; i < m - 1; i++)
+            for (int x = 1; x <= n; x++)
+                if (int p = pa[x][i]; p != -1)
+                    pa[x][i + 1] = pa[p][i];
+    }
+
+    int get_kth_ancestor(int node, int k) {
+        for (; k; k &= k - 1)
+            node = pa[node][__builtin_ctz(k)];
+        return node;
+    }
+
+    int query(int x, int y) {
+        if (depth[x] > depth[y])
+            swap(x, y);
+        y = get_kth_ancestor(y, depth[y] - depth[x]);
+        if (y == x)
+            return x;
+        for (int i = pa[x].size() - 1; i >= 0; i--) {
+            int px = pa[x][i], py = pa[y][i];
+            if (px != py) {
+                x = px;
+                y = py;
+            }
+        }
+        return pa[x][0];
+    }
+};
+