Freaks City

In-Order Traversal Recursive List<Integer> inorder(TreeNode node) { List<Integer> out = new ArrayList<>(); walk(node, out); return out; } void walk(TreeNode cur, List<Integer> acc) { if (cur == null) return; walk(cur.left, acc); acc.add(cur.val); walk(cur.right, acc); } Iterative (Single Sta ...

The Confounding Switches Time Limit: C/C++ 1000MS, Other Languages 2000MS Memory Limit: C/C++ 256MB, Other Languages 512MB Description Have you ever played the "Light Switch" game? There are \(25\) lights arranged in a \(5×5\) grid. Each light has a switch that can toggle its state. In each move, a player can toggle one light's sta ...

Suppose you are given an integer array arr. The goal is to split it into the fewest possible subsets such that every pair of elements inside the same subset is coprime (their greatest common divisor equals 1). We can solve this problem using a DFS backtracking approach. Below are two distinct strategies, each corresponding to a different way of ...

Recursion can be categorized into path-aware and path-unaware forms. Most tree-related recursions are path-aware. Fundamentally, recursion implements depth-first search (DFS). To solve problems recursively, treat the recursive function as a black box that handles a subproblem, then reuse it. Beginners often try to fully expand the recursion, wh ...

The word chain problem involves constructing the longest possilbe sequence ("dragon") from a given set of words, starting with a specified character. Each word may be used at most twice in the chain. When two words are joined, overlapping parts are merged into one—e.g., beast and astonish form beastonish. Importantly, no word in the c ...

Problem 216: Combination Sum III Description: Given two integers k and n, find all possible combinations of k numbers from 1 to 9 that add up to n. Each number can only be used once in a combination. Approach This problem requires finding subsets of size k from the set [1,2,3,4,5,6,7,8,9] where the sum equals n. The parameter k represents the d ...

Introduction During the summer vacation, I systematically studied various operations on trees through Teacher Tuo's sharing and discovered many useful techniques, which I now summarize. Teacher Tuo is amazing! One Problem Type: Batch processing queries of the form f(dep(lca(x,y))), where f(x) is a function of x. Approach: First perform tree ...

Depth-First Search on Adjacency-Matrix Graphs void DFS(MGraph G, Vertex v, void (*visit)(Vertex)) { visit(v); Visited[v] = true; for (Vertex w = 0; w < G->Nv; ++w) if (G->G[v][w] && !Visited[w]) DFS(G, w, visit); } Breadth-First Search on Adjacency-List Graphs void BFS(LGraph G, Vertex s, void ...

Algorithmic Approach When handling graph problems, especially those involving large datasets, an adjacency list is often preferred over an adjacency matrix to optimize memory usage, particularly when the vertex count may exceed standard limits. To determine if an undirected graph is connected, one can traverse the structure using either Depth-F ...

RC-u1 Heat Wave Problem Summary: Given daily maximum temperatures and the day of the week for the first day, count how many days have temperatures ≥ 35°C. Days falling on weekends (Saturday and Sunday) should be counted separately. Solution: Iterate through the temperature data while tracking the current weekday. For each temperature ≥ 35, incr ...