Add generic reroot code

content/graphs/Reroot.h

@@ -0,0 +1,34 @@
+/**
+ * Author: Nic Washbourne
+ * Date: 2025-09-26
+ * Source: Sachin Sivakumar
+ * Description: Generic reroot DP with decombine. The provided operations solve sum of distances to all nodes.
+ * Time: O(N)
+ * Status: Tested on CSES: Tree Distances II
+ */
+#pragma once
+
+using T = pair<ll, ll>;
+T init() { return {1, 0}; }
+T combine(T a, T b) { return {a.first + b.first,
+		a.second + b.second + b.first}; }
+T decombine(T a, T b) { return {a.first - b.first,
+		a.second - b.second - b.first}; }
+
+vector<vi> adj;
+vector<T> dp, dpr;
+
+T dfs(int u, int p) {
+	dp[u] = init();
+	for (int v : adj[u]) if (v != p)
+		dp[u] = combine(dp[u], dfs(v, u));
+	return dp[u];
+}
+
+T dfsr(int u, int p, T dpr_p = init()) {
+	dpr[u] = dp[u];
+	if (p != u) dpr[u] = combine(dpr[u], dpr_p);
+	for (int v : adj[u]) if (v != p)
+		dfsr(v, u, decombine(dpr[u], dp[v]));
+	return dpr[u];
+}

content/graphs/chapter.tex

@@ -29,6 +29,7 @@ \section{DFS algorithms}
 	\kactlimport{2sat.h}
 	\kactlimport{EulerWalk.h}
 	\kactlimport{DominatorTree.h}
+    \kactlimport{Reroot.h}
 
 \section{Coloring}
 	\kactlimport{EdgeColoring.h}