Code Snippets

On this page I plan on documenting all the code snippets I use frequently. I’ll use T for generic types, K for generic keys, and V for generic values when necessary. All of this code is written in Java since I think it’s the most user-friendly.

Maps

Frequency Map

public Map<T, Integer> frequencyMap(T[] nums) {
    Map<T, Integer> map = new HashMap<>();
    for (T num : nums) {
        map.put(num, map.getOrDefault(num, 0) + 1);
    }
    return map;
}

Map Iteration

public void iterateMap(HashMap<K, V> map) {
    for (Map.Entry<K, V> entry : map.entrySet()) {
        System.out.println(entry.getKey());
        System.out.println(entry.getValue());
    }
}

Trees

In-Order Traversal

public void inOrder(Node root) {
    if (root == null) {
        return;
    }
 
    inOrder(root.left);
    System.out.println(root.val);
    inOrder(root.right);
}

Pre-Order Traversal

public void preOrder(Node root) {
    if (root == null) {
        return;
    }
 
    System.out.println(root.val);
    preOrder(root.left);
    preOrder(root.right);
}

Post-Order Traversal

public void postOrder(Node root) {
    if (root == null) {
        return;
    }
 
    postOrder(root.left);
    postOrder(root.right);
    System.out.println(root.val);
}

Level-Order Traversal

public void levelOrder(Node root) {
    Queue<Node> queue = new LinkedList<Node>();
    queue.add(root);
 
    while (!queue.isEmpty()) {
        int size = queue.size();
        for (int i = 0; i < size; i++) {
            Node curr = queue.poll();
            System.out.println(curr.val);
            queue.add(curr.left);
            queue.add(curr.right);
        }
    }
}

Graphs

Depth-First Search

public void dfs(HashSet<Node> visited, Node root) {
    if (root == null) {
        return;
    }
 
    visited.add(root);
    for (Node child : root.children) {
        if (!visited.contains(child)) {
            dfs(visited, child);
        }
    }
}
 

This can be done iteratively with a stack instead of recursively.

Breadth-First Search

public void bfs(Node root) {
    HashSet<Node> visited = new HashSet<>();
    Queue<Node> queue = new LinkedList<Node>();
    queue.add(root);
 
    while (!queue.isEmpty()) {
        Node curr = queue.poll();
        for (Node child : curr.children) {
            if (!visited.contains(child)) {
                queue.add(child);
                visited.add(child);
            }
        }
    }
}

Level-Order Traversal

public void levelOrder(Node root) {
    HashSet<Node> visited = new HashSet<>();
    Queue<Node> queue = new LinkedList<Node>();
    queue.add(root);
 
    int level = 0;
    while (!queue.isEmpty()) {
        int size = queue.size();
        for (int i = 0; i < size; i++) {
            Node curr = queue.poll();
            for (Node child : curr.children) {
                if (!visited.contains(child)) {
                    queue.add(child);
                    visited.add(child);
                }
            }
        }
        level++;
    }
}

Level-order traversals differ from BFS because they keep track of what “step” of the BFS we’re currently on. This can be handy for problems where we want to know the length of the shortest path, like Word Ladder or Rotting Oranges.

Matrices

Note that matrices are a subset of graphs.

Number of Islands

public void numberOfIslands(int[][] grid) {
    int numIslands = 0;
    for (int i = 0; i < grid.length; i++) {
        for (int j = 0; j < grid[0].length; j++) {
            if (grid[i][j] == 1) {
                numIslands++;
                dfs(grid, i, j);
            }
        }
    }
}
 
public void dfs(int[][] grid, int i, int j) {
    if (i < 0 || i >= grid.length || j < 0 || j >= grid[0].length || grid[i][j] == 0) {
        return;
    }
 
    grid[i][j] = 0;
    dfs(grid, i + 1, j);
    dfs(grid, i - 1, j);
    dfs(grid, i, j + 1);
    dfs(grid, i, j - 1);
}

This is one of my favorite paradigms since it applies to so many different problems.

Cool Stuff

YouTube Video Downloader

import pytube
 
video_url = 'https://www.youtube.com/watch?v=dQw4w9WgXcQ'
pytube.YouTube(video_url).streams.first().download()