338. Familystrokes May 2026

Proof. By definition a leaf has no children, thus rule 1 (vertical stroke) and rule 2 (horizontal stroke) are both inapplicable. ∎ Every internal node (node with childCnt ≥ 1 ) requires exactly one vertical stroke .

if childCnt > 0: // v has at least one child → internal internalCnt += 1 if childCnt >= 2: horizontalCnt += 1 338. FamilyStrokes

Memory – The adjacency list stores 2·(N‑1) integers, plus a stack/queue of at most N entries and a few counters: O(N) . if childCnt > 0: // v has at

while stack: v, p = stack.pop() child_cnt = 0 for w in g[v]: if w == p: continue child_cnt += 1 stack.append((w, v)) if child_cnt: internal += 1 if child_cnt >= 2: horizontal += 1 ∎ answer = internalCnt + horizontalCnt computed by

1 if childCnt(v) = 1 2 if childCnt(v) ≥ 2 0 if childCnt(v) = 0 Proof. Directly from Lemma 2 (vertical) and Lemma 3 (horizontal). ∎ answer = internalCnt + horizontalCnt computed by the algorithm equals the minimum number of strokes needed to draw the whole tree.

Both bounds comfortably meet the limits for N ≤ 10⁵ . Below are clean, self‑contained implementations in C++17 and Python 3 that follow the algorithm exactly. 6.1 C++17 #include <bits/stdc++.h> using namespace std;

Proof. If childCnt ≥ 2 : the children occupy at least two columns on the next row, so a horizontal line is needed to connect the leftmost to the rightmost child (rule 2).